MCP servers for physical mail, compared (checked 2026-08-13)

When we first wrote this page there were four. On 2026-08-13 the official MCP Registry alone lists FrankKi, SendLetter, MailStream, mailbox.bot, Mailsnail and a third-party PixelLetter wrapper; PostalForm runs its own server outside the registry; LetterAPI ships a local one; Sendoso runs an enterprise beta. Physical mail stopped being an empty category some time ago.

We build one of these products, so read the table first and the prose second. Every provider block below carries its own Checked: date. There is no blanket stamp, because a blanket stamp is a lie the moment one vendor ships something. Rows dated 2026-08-13 were re-fetched from that vendor's own pages on 2026-08-13. Rows dated 2026-08-04 were not re-fetched and say so.

A cell reading not measured means no page we fetched stated it. It does not mean the feature is absent. We make no exclusivity claim anywhere on this page: several of these products overlap heavily, and more than one does something FrankKi does not.

Vendor pages change without notice. If a figure here disagrees with a vendor's own site today, the vendor's site wins.

The short answer

• A letter into or across Germany, Austria and Switzerland with German postal formalities: FrankKi. A letter anywhere in 15 named European countries at a published VAT-inclusive price: SendLetter, at EUR 2.36 into Germany. For a one-off sender that is the cheapest published route for a plain standard letter. It stops being the cheapest into Germany at 20 letters a month, where a FrankKi partner account is EUR 2.19 or less, against EUR 2.49 below that. Our figures are VAT-inclusive too.
• Mail that has to survive a compliance review (proof of posting, a named registered product or an explicit refusal, client separation, human approval before dispatch): FrankKi. This is a workflow claim, not a coverage claim.
• US sender and US recipient, price deciding: Mailsnail at $1.50 per first-class letter and mailbox.bot at $1.00 flat for a one-page letter were the lowest published per-letter prices we saw on 2026-08-13. MailStream's $99/month wholesale tier goes lower per piece at volume.
• An agent that also needs to receive and read incoming mail: mailbox.bot. Nobody else in this comparison documented inbound scanning and OCR.
• Machine payments with no account at all: PostalForm (MPP, x402, Stripe Agentic Commerce, no API key for agent flows). Mailsnail still keeps an anonymous 402 route, but as of 2026-08-18 its own docs demote it: the managed server signs in and draws on a prepaid balance, and it says "Most coding agents cannot do this today" of the anonymous one. (On 2026-08-13 this entry read "Mailsnail (Stripe Link Shared Payment Token, no signup)".)
• Postcards: MailStream, Mailsnail. FrankKi sends letters, not postcards.
• Enterprise gifting and B2B direct mail inside a revenue workflow: Sendoso's MCP beta.
• High-volume US direct mail with an established vendor: Lob or PostGrid, and accept that the agent layer is a third-party aggregator, not the vendor's own server.

Comparison table

Prices are per letter unless stated. Currency and check date are on every price.

	FrankKi	SendLetter	MailStream	mailbox.bot	PostalForm	Mailsnail	LetterAPI
Registry name	app.frankki/letters 1.0.0	eu.sendletter/sendletter 1.0.0	app.mailstream/mailstream 1.0.0	bot.mailbox/mailbox 1.0.1	not in the registry on 2026-08-13	io.github.iancollins27/mailsnail 0.6.0	not in the registry on 2026-08-13
Endpoint	https://mcp.frankki.app	https://sendletter.eu/api/mcp (registry); the vendor page gives https://sendletter.eu/mcp	https://my.mailstream.app/mcp	https://mailbox.bot/api/mcp	/mcp on postalform.com, streamable HTTP	https://api.mailsnail.dev	local only: npx @letterapi/mcp
Home market	Germany / DACH	Europe	United States	United States	United States	United States	Canada + United States
Countries	DACH-oriented; worldwide destination quoting conditional on country policy, an open shipping zone, Pingen acceptance and a successful price. shipping_quote decides	15 named: NL, AT, BE, CH, DE, DK, ES, FR, GB, IE, IT, LU, PL, PT, SE, plus a Rest-of-World tier	United States, USPS	United States; carrier list names USPS, FedEx, UPS, DHL and others. No country list published	12 named: US, CA, AT, BE, CH, DE, ES, FR, GB, IN, LU, NL	United States, USPS	Canada, USA
Registered mail	Einschreiben Einwurf and Übergabe as named catalog products; where a destination has no supported product the call is refused with LEGAL_PROOF_UNAVAILABLE_FOR_COUNTRY rather than quietly downgraded	Yes, across the 15 countries: carrier track and trace plus a PDF with contents, address and time of posting	not measured	US Certified Mail	US First Class certified only, with optional Electronic Return Receipt. No other country named for certified mail	US Certified letter	not measured
Auth	OAuth 2.1, Authorization Code, PKCE S256, Dynamic Client Registration live, 21 scopes. Scoped Bearer API key for headless. Public signup open	OAuth sign-in; send_letter additionally needs explicit approval, a single-use confirmation token and a stable idempotency key	OAuth 2.1 with automatic client registration; no API key required	Account with API key; sandbox and live keys	No API key for agent flows; humans use hosted checkout	OAuth sign-in, and sends draw on a prepaid balance; an anonymous 402 route with a Stripe Link Shared Payment Token remains, which the vendor itself calls unusable for most agents. On 2026-08-13 this read "No signup"; re-fetched 2026-08-18	Token in the Authorization header, no Bearer prefix, no OAuth
Tools	72 total, 8 app-only, 64 model-visible (39 read-only, 25 action)	9: price, address validation, draft, preview, confirmation, send, status, list, cancel	27	not measured	not measured; drafts, campaigns, checkout and status are named	7: preview_letter, send_letter, send_postcard, verify_address, get_letter, list_letters, cancel_letter	4: send_letter, get_order, list_orders, get_account
Sandbox / dry run	Sandbox is a per-account grant and is off by default. Our own production probe on 2026-08-13 returned sandbox: false, status unavailable. The documented substitute is dryRun: true, which validates and previews with nothing printed and nothing charged	Sandbox account runs the whole chain without printing or charging	Sandbox enabled by default for all accounts	Sandbox keys and sandbox tests	not documented on the page	preview_letter returns a proof PDF and price without charging or mailing	Dev mode with test keys, up to 3 addresses per call
Spend caps	order_send aborts above maxCostEuros, and aborts with PRICE_CHANGED on a priceVersion mismatch	not measured	not measured	max-cost headers per request	not measured	not measured	not measured
Approval before dispatch	Default mode review: every send waits for a human, and expires after 10 minutes with nothing charged	Explicit approval plus a single-use confirmation token	not measured	requires_approval=true	Hosted checkout is the recommended human step; no named approval API	None. The vendor's own framing is "no humans in the loop"	not measured
Machine payments	Prepaid wallet funded through hosted Stripe Checkout; order_send debits it. The MCP chat never collects card data	Free account, pay per letter	Subscription or pay-as-you-go	not measured	MPP, x402, Stripe Agentic Commerce	Stripe Link Shared Payment Token per transaction, automatic refund if the mailing fails	not measured
Price	Server-authoritative, quote before send. MCP partner tariff: from EUR 1.49 b/w, from EUR 1.59 colour; Einwurf from EUR 5.59 (entry tier EUR 6.59); Übergabe from EUR 5.99 (entry tier EUR 6.99). The volume ladder starts at 20 letters a month, runs through EUR 2.19, 1.99 and 1.79, and reaches its floor at 1,000 German standard letters a month; below 20 letters a month you pay EUR 2.49. The rung you reach applies to every letter that month, including the first. Without a partner account (iOS app, web checkout): from EUR 3.39 b/w, EUR 3.49 colour	EUR incl. VAT, 1 page: standard 2.36–4.48, priority 3.57–4.78, registered 12.04–47.92, extra page 0.24. Germany 2.36 / 3.57 / 12.04	USD: $99/month flat for wholesale, or pay-as-you-go letters from $0.97, postcards from $0.82; with the subscription letters from $0.80, postcards from $0.58	USD: one-page USPS First-Class letter $1.00 flat	USD: base fee plus per-page printing, postage included; one-page b/w First Class from about $3.40. The vendor says the checkout quote is the source of truth	USD: first-class letter $1.50, certified $9.75, 4×6 postcard $1.00. Certified was $9.00 on 2026-08-13; re-fetched 2026-08-18	USD: $2.49 to Canada, $3.25 to USA, $1.80 at 500+ letters/month
Inbound mail	Not part of the product	not measured	not measured	Yes: scans, OCR, extracted fields; package workflows in the managed receiving-address product	not measured	not measured	not measured
Checked	2026-08-13	2026-08-13	2026-08-13	2026-08-13	2026-08-13	2026-08-13	2026-08-13

What each one does well

SendLetter: published European prices, and a send that cannot happen by accident

Checked: 2026-08-13. SendLetter published to the registry on 2026-08-08 as eu.sendletter/sendletter and is the most direct European comparison to FrankKi on this page. It names 15 countries — Netherlands, Austria, Belgium, Switzerland, Germany, Denmark, Spain, France, United Kingdom, Ireland, Italy, Luxembourg, Poland, Portugal, Sweden — plus a Rest-of-World tier, and it publishes a full price table in EUR including VAT rather than making you call an API to find out.

Its send safety model is worth studying regardless of which product you pick. Nine tools keep pricing, address validation, drafting, preview, confirmation, sending, status, listing and cancellation apart from each other, and send_letter needs three separate things at once: explicit approval, a single-use confirmation token issued by the server after revalidation, and a stable idempotency key. The vendor's own sentence is the clearest statement of the idea anyone in this category has published: the agent can prepare a draft, but only a server token issued after revalidation can authorise real mail.

Its sandbox account runs the whole chain without printing or charging. On that specific axis SendLetter is ahead of us today, and we would rather say so than have you find out yourself.

Source: sendletter.eu, sendletter.eu/pricing, sendletter.eu/en/integrations/mcp, and the official MCP Registry. Checked 2026-08-13.

MailStream: the widest US tool surface, and a claim worth dating

Checked: 2026-08-13. MailStream published on 2026-07-20 and exposes 27 tools at https://my.mailstream.app/mcp, covering postcards, letters, address verification and campaign management with analytics. Auth is OAuth 2.1 with automatic client registration and no API key. Sandbox is on by default for every account, which is the friendliest default in this comparison.

Its pricing is the only genuine subscription model here: $99 per month flat buys wholesale rates, or you pay as you go from $0.97 per letter and $0.82 per postcard. Coverage is the United States via USPS.

One thing to date rather than dispute. MailStream's developer page states there are 12,000+ MCP servers and "zero are for physical mail, except this one", and its registry description calls it "the first MCP server for physical mail". On 2026-08-13 the same registry that carries MailStream's own entry also carries mailbox.bot, published 2026-05-27 — nearly two months earlier — alongside Mailsnail, a PixelLetter wrapper, SendLetter and us. We are not going to characterise anyone's intent; we are just going to note that the registry is public and both dates are in it.

Source: mailstream.app/developers/mcp-server, lp.mailstream.app/api, and the official MCP Registry. Checked 2026-08-13.

mailbox.bot: the only one here that reads your post as well as sending it

Checked: 2026-08-13. mailbox.bot is the oldest entry in the registry for this category, published 2026-05-27. Outbound it does US letters and Certified Mail at $1.00 flat for a one-page USPS First-Class letter, with requires_approval=true for an approval gate and max-cost headers for per-request spending limits — a control pair nobody else on this page ships together.

Inbound is where it stands alone in this comparison: scans, OCR and extracted fields, with eligible package workflows part of a managed receiving-address product. That product is in a reservation phase today — reservations are open, identity verification and facility approval are required, and the monthly price is planned at $15 for approved accounts. The page did not state an issuing date, so neither do we.

Its tool count was not restated on the pages we fetched, so we are not going to print a number for it.

Source: mailbox.bot and the official MCP Registry. Checked 2026-08-13.

PostalForm: the most complete machine-payment story

Checked: 2026-08-13. PostalForm runs a streamable HTTP MCP server at /mcp on its own domain. A GET to https://postalform.com/mcp returned HTTP 405 on 2026-08-13, which is what a POST-only JSON-RPC endpoint should do. It names 12 destinations: US, CA, AT, BE, CH, DE, ES, FR, GB, IN, LU, NL. Agent flows need no API key at all; MPP, x402 and Stripe Agentic Commerce carry the payment, with hosted checkout as the human path.

Two things to read carefully if you are in Europe. First, certified mail is described for qualifying U.S. First Class mail only, with an optional Electronic Return Receipt; no other country is named for certified or registered mail on the page we fetched today, Germany included. If your reason for sending is that you need documented proof of posting inside Germany, that gap is the whole answer. Second, no sandbox was documented on the page — which does not prove one does not exist, only that we could not find it.

Pricing is a base fee plus per-page printing with postage included, from about $3.40 for a one-page black-and-white First Class letter, and the vendor states plainly that the checkout quote is the source of truth. We agree with that principle and apply it to ourselves.

Source: postalform.com/ai and postalform.com/mcp. Checked 2026-08-13.

Mailsnail: sign in, prepaid balance, and an anonymous route that still exists

Re-checked: 2026-08-18. Mailsnail changed its primary model between our two fetches, and the change runs against how this page first described it. Its llms.txt now opens: "Add the remote server, sign in from your client, and mail draws against a prepaid balance - or self-host it against your own Click2Mail or Lob account." Sign-in is OAuth: the 401 challenge points the client at the authorization server, and it is email and password today. The vendor states its reason plainly, that a fixed per-charge processing fee would be a fifth of the price of a $1.50 letter.

The no-account route has not been removed. There is still a section headed "Anonymous HTTP API (no account)", where POST /v1/letters without payment returns 402 with a payment_request settled by a Stripe Shared Payment Token. Mailsnail's own caveat is the part that matters: "Requires a client that can mint a Stripe Shared Payment Token. Most coding agents cannot do this today - use the managed server above instead."

Seven tools, US-only via USPS, free USPS CASS address validation through verify_address, and preview_letter returns a proof PDF and price without charging or mailing. If the mailing fails, the refund is automatic.

Published prices, re-fetched 2026-08-18: $1.50 for a first-class letter, $9.75 for a certified letter, $1.00 for a 4×6 postcard. 6×9 and 6×11 postcards are offered without a published price. On 2026-08-13 the certified letter was $9.00; the other two are unchanged.

What this page said on 2026-08-13: that Mailsnail was "the purest expression of the agentic idea on this page" because "there is no signup", and that a Stripe Link Shared Payment Token was the whole authentication story. That was true of the service we fetched then and is no longer true of the primary path. It is corrected here rather than deleted, because a comparison that quietly rewrites its own history is worth nothing.

If your use case is a US-domestic letter fired by an autonomous agent with nobody watching, this is still a short path, provided your client can either sign in or mint a Shared Payment Token. No amount of German compliance tooling makes us a better fit for that shape.

Source: mailsnail.dev and the official MCP Registry. Prices and payment model re-fetched 2026-08-18, other rows checked 2026-08-13.

LetterAPI: North America, four tools, no ceremony either

Checked: 2026-08-13. LetterAPI is a local stdio server started with npx @letterapi/mcp --key ltr_live_xxxx. Four tools, token in the Authorization header with no Bearer prefix and no OAuth, and coverage limited to Canada and the USA. Dev mode with test keys allows up to three addresses per call with no real mailings. Published prices: $2.49 per letter to Canada, $3.25 to the USA, and $1.80 at 500+ letters per month with custom branding, invoice billing and an SLA.

Source: letterapi.io. Checked 2026-08-13.

PixelLetter: a German option, wrapped by someone else

Checked: 2026-08-13. The registry entry io.github.bitterdev/pixelletter-mcp (1.0.3, published 2026-07-27) is an npm package with stdio transport that wraps PixelLetter's interface, with test mode included. Read the namespace: io.github.bitterdev is an independent developer, not PixelLetter GmbH & Co. KG. pixelletter.de made no mention of MCP on 2026-08-13.

PixelLetter itself sends letters, postcards, faxes and Einschreiben from Germany. We could not verify its prices today — its price page returned HTTP 404 and the homepage publishes none — so we are not printing any. Nor could we confirm the dispatch-centre count sometimes attributed to it.

If you want a German-language mail service driven from an agent and you are comfortable running a community-maintained local wrapper, this is a real option. What you do not get from a third-party wrapper is a vendor who owns the agent surface end to end.

Source: pixelletter.de and the official MCP Registry. Checked 2026-08-13.

Sendoso: a different category, listed so you do not miss it

Checked: 2026-08-13. Sendoso announced an MCP beta on 2026-06-24 with founding integration partners Demandbase, UserGems, Gong, Outreach, Clay and Warmly, positioned around a signal-to-send-to-follow-up revenue workflow for marketing, sales and customer-success agents. Access is request-based and approved on a rolling basis, so we could not inspect the tool surface, auth, coverage or pricing — all of that is not measured here.

This is enterprise gifting and B2B direct mail, not a per-letter transactional API. If your question is "can an agent send a gift and a note into a live deal", it is the answer on this page; if your question is "what does one letter to a German authority cost", it is not.

Source: Sendoso's 2026-06-24 MCP beta announcement and sendoso.com. Checked 2026-08-13.

FrankKi: German and DACH workflows with conditional worldwide quoting

Checked: 2026-08-13 against our own production server. FrankKi MCP is a live remote hosted Model Context Protocol server for physical mail. You add https://mcp.frankki.app to Claude, ChatGPT, Cursor, VS Code or your own agent, and 64 model-visible tools let it draft, price, send, track and manage a real letter without leaving the chat. FrankKi is the agentic layer of letters: physical mail for AI agents and people, on one fulfillment path.

Where FrankKi differs, and where it does not

Four axes below are documented in FrankKi source and were verified against the production server on 2026-08-13. None of them is an exclusivity claim. Several providers on this page do things we do not.

1. German and DACH workflows, with destination-checked worldwide delivery

FrankKi is built on German postal and business norms: DIN 5008 typesetting, destination-supported registered products, EUR pricing, GoBD-oriented records and DATEV-compatible export. Its source supports worldwide delivery through Pingen only when the destination is admitted by current country policy, the shipping zone is open, and Pingen accepts and successfully prices the request. International carrier, product, price and timing are destination-specific; call shipping_quote before sending, and treat an unavailable quote as unavailable rather than guessing.

Europe is contested now. SendLetter publishes standard letters into Germany at EUR 2.36 including VAT, and PostalForm lists Germany, Austria and Switzerland among its destinations. If your requirement is simply "a letter arrives in Germany", you have several routes and price is a fair way to choose. Our documented difference is what happens around the letter, not that the letter arrives.

2. Proof, and a refusal instead of a silent downgrade

Einschreiben Einwurf gives you an Einlieferungsbeleg (proof of posting) from EUR 5.59 on the MCP partner tariff (entry tier EUR 6.59). Einschreiben Übergabe is handed over against signature from EUR 5.99 (entry tier EUR 6.99). order_einlieferungsbeleg returns the document to the agent.

The part that matters more than either product: where a destination has no supported legal-proof product, FrankKi refuses the call with LEGAL_PROOF_UNAVAILABLE_FOR_COUNTRY. It does not quietly send an ordinary letter and let you find out in court. SendLetter offers registered mail across 15 European countries and mailbox.bot, PostalForm and Mailsnail all offer US certified mail — coverage is not the differentiator. The refusal behaviour is.

3. An agent you can hand to a compliance officer

Mandanten (client matters) keep one client's correspondence separate from another's, which is what a law firm or a tax advisor actually needs. Approval flows put a human decision in front of dispatch: the default mode is review, every send waits, and an undecided send expires after 10 minutes with nothing charged. Read tools are separated from action tools by each tool's mutating flag, and order_send aborts above maxCostEuros or with PRICE_CHANGED if the price version moved under it.

We verified all of that against the production server on 2026-08-13, including the 10-minute expiry.

The honest framing: this is not a longer feature list, it is a different buyer. If your legal or compliance team has to sign off before an agent touches outbound mail, this axis is the whole decision. If nobody has to sign off, PostalForm with machine payments and no API key will get you there faster; on Mailsnail the primary path has gone through a sign-in since 2026-08-18.

4. Where we are behind: the sandbox

This is the row we would most like to be able to write differently. FrankKi's sandbox is a per-account grant and it is off by default. Our own production health probe on 2026-08-13 returned capabilities.sandbox: false and reported the sandbox as unavailable for that account. The documented substitute is dryRun: true, which validates a letter fully and returns a preview with nothing printed and nothing charged, and letter_create_draft is free, so an agent can compose and price a letter it never sends.

That is a real capability and for most integration testing it is enough. It is still not the same thing. SendLetter's sandbox runs the entire chain, MailStream's is on by default for every account, and mailbox.bot ships sandbox keys. If a full sandbox before your first live send is a hard requirement, say so when you sign up rather than assuming it is switched on.

5. Prepaid wallet instead of card collection in the MCP chat

With a partner account (dashboard, MCP, API) a German standard letter starts at EUR 1.49 in black and white and EUR 1.59 in colour. The volume ladder is all-units over a trailing 30 days, so the rung you reach applies to every letter that month, including the first. It starts at 20 letters a month, runs through EUR 2.19, 1.99 and 1.79, and reaches its floor at 1,000 letters a month; below 20 letters a month you pay EUR 2.49. Registered mail starts at EUR 5.59 for Einwurf-Einschreiben (entry tier EUR 6.59) and EUR 5.99 for Übergabe-Einschreiben (entry tier EUR 6.99). Without a partner account, in the iOS app or the web checkout, a letter is from EUR 3.39 in black and white or EUR 3.49 in colour. No subscription, no minimum term. shipping_quote returns the exact price before any send. MCP partners fund their prepaid wallet through a hosted Stripe Checkout page; order_send debits the wallet. The MCP chat never collects card data.

Prices are server-authoritative. Never hardcode one into an agent: quote, then send.

Lob alternative with worldwide delivery

Checked: 2026-08-13. Lob has no first-party MCP server. On 2026-08-13 https://www.lob.com/mcp returned HTTP 404, https://www.lob.com/developers documented API documentation and SDKs while naming no MCP server and no AI-agent integration, and a search of the official MCP Registry returned no Lob direct-mail entry.

Agent access to Lob runs through third-party aggregators. MCPBundles publishes a managed Lob bundle at https://mcp.mcpbundles.com/bundle/lob exposing 49 tools plus a skill file, authenticating with your own Lob API key. MCPBundles operates it; Lob does not. That is a workable route, but it means your agent's mail surface is maintained by a party with no commercial relationship to your postage.

If you are comparing Lob with FrankKi for an agent workflow, FrankKi provides a first-party hosted MCP endpoint and conditional worldwide destination quoting without requiring an aggregator or your own wrapper. Confirm the destination with shipping_quote.

Lob's own product range, geography and pricing were not rechecked on 2026-08-13 and nothing dated is restated here. For high-volume US direct mail, Lob remains a serious vendor and this page does not argue otherwise.

Source: lob.com/mcp (404), lob.com/developers, mcpbundles.com/skills/lob, and the official MCP Registry. Checked 2026-08-13.

PostGrid alternative for Europe and international mail

Checked: 2026-08-13 for MCP availability only. A search of the official MCP Registry on 2026-08-13 returned no PostGrid direct-mail entry, and no first-party PostGrid MCP server was found. We did not verify any specific third-party PostGrid wrapper today, so wrapper availability is not measured rather than asserted.

PostGrid's address-verification reach, its product range and its pricing were not rechecked on 2026-08-13. An earlier review on 2026-08-04 recorded an advertised address-verification claim covering 245+ countries with a multilingual parser; that is a dated claim about address verification, not physical delivery, and it has not been revisited.

If you need an agent that mails into Europe with European postal products rather than an international lane, FrankKi and SendLetter both cover that natively. If you need bulk address verification across a long tail of countries, PostGrid is the better tool and the two are not really competing.

Source: official MCP Registry, checked 2026-08-13. PostGrid product claims: last checked 2026-08-04.

Click2Mail

Checked: 2026-08-04. Not revisited on 2026-08-13. The Click2Mail developer pages reviewed on 2026-08-04 described USPS print-and-mail products and published developers.click2mail.com/llms.txt as a documentation index. A first-party Click2Mail MCP server was not found in those reviewed pages. That is a dated, scoped search result, not proof of current absence, and it has not been rechecked.

Source: developers.click2mail.com, last checked 2026-08-04.

When you do not need FrankKi

We would rather you pick the right tool than churn in a month. Choose something else if:

• Your sender and recipient are both in the United States and price per letter decides. mailbox.bot at $1.00 for a one-page letter and Mailsnail at $1.50 were the lowest published per-letter prices on 2026-08-13; MailStream's $99/month wholesale tier goes lower at volume.
• You want a plain European letter at the lowest published price and you send few of them. SendLetter lists a standard letter into Germany, Spain or the Netherlands at EUR 2.36 including VAT. Below 20 letters a month our German standard letter is EUR 2.49, also including VAT, so SendLetter is cheaper. From 20 letters a month ours is EUR 2.19 or less into Germany and the order reverses; for Spain or the Netherlands ask shipping_quote, because the ladder price quoted here is a German standard letter.
• Your agent has to pay with no human in the loop at all. FrankKi puts every send into review by default and a human has ten minutes to release it. PostalForm takes machine payments with no API key. Mailsnail keeps an anonymous 402 route, but since 2026-08-18 its managed server signs in and draws on a prepaid balance, and it says itself that most coding agents cannot use the anonymous one. (This line read "Mailsnail requires no signup and no human" on 2026-08-13.)
• You need a full sandbox before your first live send. SendLetter and MailStream have one on by default; ours is a per-account grant that is off by default.
• You need to receive mail, not just send it. mailbox.bot does inbound scanning, OCR and extracted fields. We do not do inbound at all.
• You want postcards. FrankKi sends letters. MailStream and Mailsnail send postcards.
• You are sending marketing volume into US households. Lob and PostGrid are built for that scale, via an aggregator MCP surface.
• You want enterprise gifting inside a revenue workflow. Sendoso's MCP beta is aimed exactly there.

Choose FrankKi when you need German and DACH formatting, a named registered product with an explicit refusal where there is none, client separation, and a human approval gate in front of every dispatch — and confirm every destination and product with shipping_quote.

How to connect FrankKi MCP

FrankKi MCP is live. Public signup without manual account approval is open at https://frankki.app/dashboard/registrieren; password setup through an email link is required, and you then create your scoped Bearer API key in the dashboard. Explicit rollout switches may disable signup.

The official MCP Registry name is app.frankki/letters.

There are two connection paths:

• Interactive OAuth path: For an interactive OAuth-capable MCP client, enter https://mcp.frankki.app as the server URL. OAuth 2.1 with Authorization Code, PKCE (S256) and Dynamic Client Registration (DCR, RFC 7591) is implemented and live in production; a GET to /.well-known/oauth-authorization-server on the MCP host returned HTTP 200 on 2026-08-13, carrying a registration endpoint, S256 as the only supported code challenge method and 21 scopes. A client that supports MCP/OAuth discovery can therefore register itself and complete the flow without a manually issued key.
• Headless or manual path: For a headless agent or manual setup, create a scoped Bearer API key in the dashboard and set Authorization: Bearer <your-api-key> while connecting to https://mcp.frankki.app.

Then call letter_create_draft followed by shipping_quote and order_send. Every physical dispatch enters human approval before postal handoff and expires after ten minutes if nobody decides. Sandbox is a per-account grant that is off by default; use dryRun: true to validate and preview a send without printing or charging anything.

FAQ

What is the best MCP server for physical mail?

It depends on the destination and the workflow, and on 2026-08-13 there is no single answer. For letters into or across Germany, Austria and Switzerland where documented proof of posting, client separation and a human approval gate matter, FrankKi. For the lowest published European price on a plain standard letter below 20 letters a month, SendLetter. For US domestic price, mailbox.bot or Mailsnail. For inbound mail, mailbox.bot. For machine payments with no account, PostalForm; Mailsnail still has an anonymous 402 route but told us on 2026-08-18 that most coding agents cannot use it, and its managed server now signs in against a prepaid balance. For enterprise gifting, Sendoso.

How many MCP servers can send physical mail?

At least eight on 2026-08-13. The official MCP Registry lists FrankKi, SendLetter, MailStream, mailbox.bot, Mailsnail and a third-party PixelLetter wrapper; PostalForm and LetterAPI ship servers outside the registry; Sendoso runs a request-based enterprise beta. Any claim that the category has one or zero entrants is out of date.

Which MCP servers let an AI agent send physical mail internationally?

FrankKi supports worldwide destination quoting subject to current country policy, an open shipping zone, Pingen acceptance and successful pricing. SendLetter names 15 European countries plus a Rest-of-World tier. PostalForm names 12 destinations including the US, Canada, Germany, Austria, Switzerland, France, Spain, the UK, India, Belgium, Luxembourg and the Netherlands. LetterAPI covers Canada and the USA. MailStream, mailbox.bot and Mailsnail described US delivery. All checked 2026-08-13.

Can an AI agent send registered mail?

Yes, from several providers. FrankKi offers German Einschreiben Einwurf with a proof-of-posting record from EUR 5.59 on the MCP partner tariff (entry tier EUR 6.59) and Übergabe from EUR 5.99 (entry tier EUR 6.99), retrievable through order_einlieferungsbeleg, and refuses with LEGAL_PROOF_UNAVAILABLE_FOR_COUNTRY where a destination has no supported product rather than downgrading silently. SendLetter offers registered mail across its 15 European countries at EUR 12.04–47.92 including VAT. mailbox.bot and Mailsnail offer US Certified Mail, Mailsnail at $9.75 (it was $9.00 on 2026-08-13; re-fetched 2026-08-18). PostalForm names certified mail for qualifying US First Class only. Other rows checked 2026-08-13.

How much does it cost to send a letter from an AI agent?

Published prices on 2026-08-13: mailbox.bot $1.00 for a one-page US letter; Mailsnail $1.50; MailStream from $0.97 pay-as-you-go or $0.80 on a $99/month plan; LetterAPI $2.49 to Canada and $3.25 to the USA; SendLetter EUR 2.36–4.48 including VAT for a European standard letter; PostalForm from about $3.40; FrankKi on the MCP partner tariff from EUR 1.49 for a German standard letter in black and white and from EUR 1.59 in colour, with the volume ladder starting at 20 letters a month and reaching its floor at 1,000 letters a month; below 20 letters a month it is EUR 2.49. Without a partner account, from EUR 3.39 black and white and EUR 3.49 colour. FrankKi's prices are server-authoritative: shipping_quote returns the exact destination price before any send, and an agent should never hardcode one.

Can I test before a real letter goes out?

Yes, but the defaults differ sharply. MailStream's sandbox is on by default for every account. SendLetter's sandbox account runs the whole chain without printing or charging. mailbox.bot ships sandbox keys. Mailsnail's preview_letter returns a proof PDF and price without charging. LetterAPI has a dev mode with test keys. FrankKi's sandbox is a per-account grant that is off by default — verified on our own production account on 2026-08-13 — and the documented substitute is dryRun: true, which validates and previews without printing or charging. PostalForm documented no sandbox on the page we fetched, which does not prove one is unavailable.

Do Lob, PostGrid or Click2Mail have an MCP server?

Lob and PostGrid have no first-party MCP server. On 2026-08-13, lob.com/mcp returned HTTP 404, lob.com/developers named no MCP server, and the official MCP Registry returned no entry for either vendor. Agent access to Lob runs through third-party aggregators such as the MCPBundles managed bundle, which exposes 49 tools using your own Lob API key. Click2Mail was last reviewed on 2026-08-04, when no first-party server was found; it has not been rechecked.

Is FrankKi only for German letters?

No. FrankKi supports worldwide delivery subject to current country policy, an open shipping zone, Pingen acceptance and successful destination pricing. Carrier, product and timing are destination-specific, so request shipping_quote before sending. The German part is the workflow and formatting basis, not a fixed destination list.

Sources and method

Every provider block above carries its own Checked: date. Blocks dated 2026-08-13 were re-fetched that day from the vendor's own pages and from the official MCP Registry, and every price is reproduced with its currency and that date. Blocks dated 2026-08-04 — Click2Mail, and the product-level claims about Lob and PostGrid — were not revisited and say so where they appear. We deliberately do not publish one blanket stamp: a per-row date is both more honest and easier to cite.

FrankKi's own row was verified on 2026-08-13 against the production server: the OAuth authorization-server metadata document, and a live health probe that returned the sandbox as unavailable for that account and the approval mode as review with a ten-minute expiry. Where our own production behaviour is worse than a competitor's — the sandbox — we have written that down rather than hidden it behind a source claim.

not measured means no page we fetched stated it. It does not mean a feature is unavailable. We make no exclusivity claim about any capability on this page.

If we got something about your product wrong, email our support team directly. We will correct verifiable errors and date the correction.

• Official MCP Registry: registry.modelcontextprotocol.io
• SendLetter: sendletter.eu, sendletter.eu/pricing, sendletter.eu/en/integrations/mcp
• MailStream: mailstream.app/developers/mcp-server, lp.mailstream.app/api
• mailbox.bot: product and pricing pages
• PostalForm: postalform.com/ai, postalform.com/mcp
• Mailsnail: mailsnail.dev
• LetterAPI: letterapi.io
• PixelLetter: pixelletter.de
• Sendoso: 2026-06-24 MCP beta announcement
• Lob: lob.com/mcp, lob.com/developers, mcpbundles.com/skills/lob
• PostGrid: postgrid.com, last checked 2026-08-04
• Click2Mail: developers.click2mail.com, last checked 2026-08-04
• FrankKi: mcp.frankki.app, frankki.app/en/mcp