Agent-to-agent payments over email. Two emails, any rail.

The idea: email already has identity (DKIM), payloads (MIME), threading (In-Reply-To), and federation (SMTP). The only missing dimension is value. Envelopay adds a payment proof to the envelope.

Sign up at AgentMail (free tier: 3 inboxes, 100 emails/day). Create your agent inboxes:

export AGENTMAIL_API_KEY="am_..."

# Create inboxes
curl -X POST https://api.agentmail.to/v0/inboxes \
  -H "Authorization: Bearer $AGENTMAIL_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"username": "axiomatic"}'

curl -X POST https://api.agentmail.to/v0/inboxes \
  -H "Authorization: Bearer $AGENTMAIL_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"username": "blader"}'

uv sync

uv run python demo/four_rails.py

Shows all four payment paths:

Plus a bounced payment (invalid proof → no FULFILL).

Two states. That's the whole protocol.

The X-Envelopay-Type header carries the state. The JSON MIME part carries the payload. DKIM proves provenance. In-Reply-To links the thread. The thread is the transaction log.

See SPEC.md for the full protocol specification.

From: axiomatic@agentmail.to
To: blader@agentmail.to
Subject: Review PR #417
Message-ID: <req-8f3a@agentmail.to>
X-Envelopay-Type: ORDER
DKIM-Signature: v=1; a=rsa-sha256; d=agentmail.to; ...
Content-Type: multipart/mixed; boundary="mp"

--mp
Content-Type: text/plain; charset=utf-8

Review PR #417 in github.com/axiomatic/widget

--mp
Content-Type: application/json; charset=utf-8

{
  "task": {"description": "Review PR #417", "repo": "github.com/axiomatic/widget"},
  "amount": "500000",
  "token": "USDC",
  "chain": "solana",
  "proof": {"tx": "4vJ9..."},
  "fallback": "https://cash.app/$axiomatic"
}
--mp--

From: blader@agentmail.to
To: axiomatic@agentmail.to
Subject: Re: Review PR #417
In-Reply-To: <req-8f3a@agentmail.to>
X-Envelopay-Type: FULFILL
DKIM-Signature: v=1; a=rsa-sha256; d=agentmail.to; ...

{
  "result": {"summary": "Approved with 2 comments"},
  "settlement": {"tx": "4vJ9...", "status": "confirmed"}
}

Two emails. Both DKIM-signed. Both parties hold the full record.

The fallback field carries a payment URL for counterparties without a wallet: Stripe checkout, PayPal, Cash App, Venmo — any URL that accepts money. The protocol mandates a proof, not a rail.

AGPL-3.0. If you serve this over a network, share your source.

• Sent — the user story
• Certified Mail — the semantics
• You Have Mail — the protocol argument
• No Postage — the economics
• Illegal Tender — the stack
• Proof of Trust — trust topology
• SPEC.md — protocol specification
• STACK.md — component map