Agent-to-Agent Email
Using standard SMTP email as the transport between AI agents — one agent sends a message to another agent's mailbox, and the recipient reads and acts on it. The simplest interoperable A2A protocol that already exists.
Why email works as an A2A protocol
Every agent-to-agent (A2A) protocol has to solve the same problems: addressing, delivery, authentication, threading, and retry. Purpose-built A2A protocols solve these from scratch. Email has solved them for forty years. When one agent emails another at its own mailbox, the global SMTP infrastructure handles delivery, DKIM handles authentication, Message-ID and References handle threading, and bounces handle retry — with no new infrastructure required.
The tradeoff is latency. Email is slow for the tight coordination loops some A2A scenarios need. For anything that can tolerate seconds-to-minutes of delay — and most genuinely autonomous agent workflows can — the tradeoff is worth it because email is the only A2A protocol that already works across organization boundaries.
What A2A email actually enables
Two agents on the same platform can email each other. Two agents on different platforms can email each other. A Loomal agent can email a Microsoft Copilot agent that can email a custom agent running on a Raspberry Pi in someone's basement — they're all just SMTP endpoints. No shared infrastructure, no API contract, no mutual trust beyond what email authentication already provides.
This matters for scenarios where agents cross organizational lines: a purchasing agent at one company negotiating with a sales agent at another; a logistics agent at a 3PL coordinating with a fulfillment agent at a retailer; a consumer's personal agent emailing a business's customer service agent. Any protocol that requires prior integration can't reach these cases. Email can.
Threading and structured payloads
Email's thread model (Message-ID, In-Reply-To, References) gives A2A conversations built-in state. An agent sending a follow-up doesn't need to invent a correlation ID — the thread headers already correlate. For structured data in the body, conventions are emerging: JSON Lines, MIME parts with application/json types, or inline YAML frontmatter. None of these need new infrastructure; any agent that can read email can parse them.
Attachments cover the binary payload case — an agent emails a PDF invoice, a dataset, a signed document. The recipient's email primitives expose attachments as fetchable resources, the model decides what to do with them.
How Loomal supports A2A email
Because every Loomal identity has a real DKIM-signed address, A2A email works without any extra setup. Agent A calls mail.send with Agent B's address; Agent B receives the message via mail.list_messages or webhook; Agent B replies with mail.reply and the thread stitches automatically. Both agents can be on Loomal or only one can be — the other side sees standard email and can respond through any mail provider.
For A2A scenarios within a single Loomal account, an agent can mint child identities for sub-agents it spawns and route conversations between them. Delegation chains keep the attribution clean even several hops deep.
Loomal primitives
mail.sendmail.replymail.list_messagesmail.get_threadSee it in production
More from the glossary
Agent 2FA (TOTP)
Time-based one-time password generation that lets an AI agent complete two-factor authentication on services that require it — without borrowing a human's phone or authenticator app.
Agent Vault
An encrypted credential store scoped to a single AI agent identity, holding API keys, OAuth tokens, and secrets the agent uses to authenticate with other services.
DKIM for Agents
DomainKeys Identified Mail signing applied to email sent by AI agents, so recipients can verify the message actually came from the agent's authorized domain — not a spoofed lookalike.
MCP Identity
An identity exposed through a Model Context Protocol server, so any MCP-compatible client (Claude, Cursor, Windsurf, the Claude Agent SDK) can act as that identity by calling its primitives as tools.
Build agents with their own identity.
Email, vault, and TOTP — provisioned in 30 seconds.
Get API Key — FreeLast updated: 2026-04-14