Architecture: self-hosted AI-native mailbox for agents
Design of record for OpenMail — one Go binary (smtpd/core/sender/mcp) + Postgres + object storage on a VPS. Covers inbound SMTP/MIME/threading, outbound deliverability (relay-default, self-host opt-in), data model, AgentMail-shaped v0 API + MCP server, and an MVP milestone path. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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# OpenMail — Architecture
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Self-hosted, AI-native mailbox for agents. One Go service, Postgres, and object storage on a VPS,
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fronted by Caddy for TLS. This document is the design of record; it favors an MVP that actually
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receives and sends real mail over a perfect feature set.
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## 0. The honest framing
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An agent-mailbox API is ~5% of the work. The other 95% is **mail infrastructure**:
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| Hard part | Why it's hard |
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|---|---|
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| **Inbound** | You must run an SMTP server on port 25 with MX records pointing at it, accept from arbitrary senders, survive spam/abuse, and parse messy real-world MIME. |
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| **Outbound deliverability** | Anyone can send SMTP; getting it to the *inbox* needs SPF + DKIM signing + DMARC alignment, PTR/rDNS, warmed IP reputation, feedback loops (FBLs), and bounce/complaint handling. This is the real moat. |
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| **MIME parsing** | Multipart trees, encodings, inline vs attachment, calendar parts, broken senders. |
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| **Threading** | Stitching messages into conversations from `Message-ID` / `In-Reply-To` / `References` plus subject heuristics. |
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| **Quoted-history stripping** | The `extracted_text` feature (reply content without the quoted thread) is its own parsing problem. |
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OpenMail is structured so each of these is an isolated, swappable component, and so v1 can lean on a
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relay for deliverability while the self-hosted SMTP path matures.
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## 1. Component overview
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```
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┌──────────────────────────────────────────────┐
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Internet senders ───▶ │ smtpd (inbound, port 25, go-smtp) │
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(MX → this VPS) │ → parse MIME → normalize → enqueue │
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└───────────────┬──────────────────────────────┘
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│ internal queue (Postgres LISTEN/NOTIFY or NATS)
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┌───────────────▼──────────────────────────────┐
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│ core (Go) │
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Agents / SDKs ──REST──▶ • inbox/message/thread/draft services │
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MCP clients ───MCP──▶ │ • threading, extraction, labels, search │
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│ • auth (API keys, pods), webhooks/WS │
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└──────┬───────────────────────┬───────────────┘
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│ │
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┌────────────▼─────┐ ┌───────────▼───────────────┐
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│ Postgres │ │ Object store (S3/MinIO) │
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│ metadata + FTS │ │ raw MIME + attachments │
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└──────────────────┘ └────────────────────────────┘
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▲
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┌───────────────┴──────────────────────────────┐
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Outbound to world ◀── │ sender (DKIM-sign → self-host SMTP OR relay)│
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└──────────────────────────────────────────────┘
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```
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All four roles (`smtpd`, `core` API, `sender`, MCP) ship in one Go binary with subcommands; deploy
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together or split later. Caddy terminates TLS for the HTTP API and auto-manages certs.
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## 2. Tech stack
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- **Language:** Go (single static binary; matches the deployment-simplicity goal and OpenMail's
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systems nature).
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- **HTTP:** `chi` router; `sqlc` + `pgx` for typed Postgres access.
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- **Inbound SMTP:** `emersion/go-smtp` (server) — listens on :25, hands raw bytes to the parser.
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- **MIME:** `emersion/go-message` (+ `enmime` for lenient real-world parsing); a quote-stripper
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(port of `talon`/`EmailReplyParser` heuristics) for `extracted_text`/`extracted_html`.
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- **DKIM:** `emersion/go-msgauth/dkim` for signing (outbound) and verifying (inbound).
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- **Outbound:** pluggable `Sender` interface — `smtp` (self-host, MX delivery) or `relay`
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(SES / Postmark / Resend API). Default to relay in v1 for deliverability; self-host is opt-in.
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- **Storage:** Postgres (metadata + full-text search via `tsvector`); S3-compatible object store
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(MinIO self-hosted, or AWS S3) for raw `.eml` and attachment blobs.
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- **Events:** Postgres `LISTEN/NOTIFY` for the internal inbound queue and for fanning out
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webhooks + a WebSocket stream. (Swap to NATS if volume warrants.)
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- **Deploy:** `docker compose` — `caddy`, `openmail` (one image, multiple commands), `postgres`,
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`minio`. MX + SPF/DKIM/DMARC DNS documented in `deploy/`.
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## 3. Data model (Postgres)
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Mirrors the resource shapes agent tooling already expects, so SDKs/MCP map cleanly.
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```sql
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pods (id, name, created_at) -- tenant isolation
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inboxes (id, pod_id, address UNIQUE, display_name, client_id, metadata jsonb, created_at, updated_at)
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threads (id, inbox_id, subject, last_message_id, message_count, labels text[], updated_at, created_at)
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messages (id, inbox_id, thread_id, message_id_hdr, in_reply_to, references text[],
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from_addr, to_addrs text[], cc text[], bcc text[], subject,
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preview, text, html, extracted_text, extracted_html,
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raw_object_key, -- pointer to raw .eml in object store
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labels text[], size_bytes, headers jsonb, ts tsvector, -- FTS
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created_at, updated_at)
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attachments (id, message_id, filename, content_type, size_bytes, object_key, inline bool, content_id)
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drafts (id, inbox_id, thread_id, to_addrs text[], cc, bcc, subject, text, html, send_at, client_id)
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api_keys (id, pod_id, hash, scopes text[], created_at) -- bearer tokens, store only the hash
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webhooks (id, pod_id, url, event_types text[], secret) -- HMAC-signed deliveries
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outbox (id, message_id, status, attempts, last_error, next_attempt_at) -- send queue + retries
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events (id, inbox_id, type, payload jsonb, created_at) -- audit + WS replay
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```
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Indexes: `inboxes(address)`, `messages(thread_id)`, `messages(inbox_id, created_at desc)`,
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GIN on `messages.ts` (search) and `*.labels`. Raw MIME and attachments live in object storage, not
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the DB — rows hold pointers.
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## 4. Request / data flows
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### Inbound (receive)
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1. `smtpd` accepts on :25, enforces basic anti-abuse (rate limit, SPF check, size cap), and writes
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the raw `.eml` to the object store.
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2. Parser builds the MIME tree → extracts `text`/`html`, attachments, headers; computes
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`extracted_text` (strip quoted history) and a `preview`.
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3. Threading resolves `thread_id` from `In-Reply-To`/`References` → fallback to normalized-subject +
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participants within a window.
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4. Row inserted; `LISTEN/NOTIFY` fires → webhook + WebSocket `message.received` event.
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### Outbound (send / reply)
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1. API builds the RFC 5322 message (sets `In-Reply-To`/`References` for replies), persists a
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`messages` row + `outbox` entry.
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2. `sender` DKIM-signs and dispatches via the configured backend (self-host SMTP or relay API).
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3. Bounces/complaints (SMTP DSN, or relay webhook) update `outbox.status` and emit events.
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### API surface (v1 — AgentMail-shaped)
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```
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POST /v0/inboxes create inbox
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GET /v0/inboxes / /v0/inboxes/{id} list / get
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POST /v0/inboxes/{id}/messages/send send
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GET /v0/inboxes/{id}/messages list (limit, page_token, labels)
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GET /v0/inboxes/{id}/messages/{id} get
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POST /v0/inboxes/{id}/messages/{id}/reply
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GET /v0/inboxes/{id}/threads /{id} list / get
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…drafts, webhooks, search…
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```
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Bearer auth (`Authorization: Bearer`). Keeping paths/shapes close to AgentMail's public v0 means an
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existing Go/Python/Node client (or MCP server) can target a self-hosted OpenMail by base-URL swap.
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### MCP server
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A thin MCP front-end over the same core: tools `create_inbox`, `list_messages`, `get_thread`,
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`send_message`, `reply`, `search` — so an agent (Claude Code, etc.) drives its own mailbox directly.
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## 5. Deliverability plan (the part that actually matters)
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- **v1: relay-by-default.** Send through SES/Postmark/Resend for inbox placement on day one; OpenMail
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still owns inbound, storage, threading, API. Lowest-risk way to be useful immediately.
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- **Self-host SMTP send (opt-in):** dedicated IP with PTR/rDNS, SPF (`v=spf1 ip4:… -all`), DKIM
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signing (publish the public key), DMARC (`p=quarantine` → `p=reject`), gradual IP warmup, FBL
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enrollment, and bounce/complaint suppression lists. Documented honestly as a multi-week reputation
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effort, not a flag flip.
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- **Inbound auth:** verify SPF/DKIM/DMARC on receipt; expose pass/fail in message metadata; optional
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reject/quarantine policy per inbox.
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## 6. MVP milestones
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1. **Core API + storage** — inboxes/messages/threads/drafts CRUD against Postgres + MinIO; bearer
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auth; no real mail yet (seed via an ingest endpoint). *Proves the data model + API.*
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2. **Inbound** — `go-smtp` on :25, MIME parse, extraction, threading, `message.received` webhook.
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MX a test domain at the VPS. *Proves receive.*
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3. **Outbound via relay** — `send`/`reply` through SES/Postmark with DKIM. *Proves the loop:
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an agent receives and replies.*
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4. **MCP server** — agent owns and operates an inbox end-to-end.
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5. **Self-host SMTP send + deliverability hardening** — the long tail.
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## 7. Repo layout (planned)
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```
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cmd/openmail/ # single binary: `serve`, `smtpd`, `sender`, `mcp` subcommands
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internal/core/ # inbox/message/thread/draft services
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internal/smtp/ # inbound server + MIME parse + extraction + threading
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internal/sender/ # Sender interface + smtp & relay backends + DKIM
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internal/store/ # sqlc queries, object-store client
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internal/api/ # chi HTTP handlers (v0 surface)
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internal/mcp/ # MCP server over core
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deploy/ # docker-compose.yml, Caddyfile, DNS (MX/SPF/DKIM/DMARC) notes
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db/migrations/ # SQL migrations
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```
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## 8. Open questions
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- Relay vs self-host SMTP as the *default* — relay is pragmatic, self-host is the point. Likely ship
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relay default + self-host documented.
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- How far to chase AgentMail API compatibility vs a cleaner native shape.
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- Search: Postgres FTS is enough for v1; revisit (Meilisearch/Typesense) only if needed.
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- Multi-tenancy depth — `pods` now, full RBAC later.
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# OpenMail
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A self-hosted, AI-native mailbox for agents — open-source infrastructure that gives an AI agent its
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own real email inbox (receive, parse, thread, search, send) on a box you control.
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Think "Gmail for agents, self-hosted." A single Go service + Postgres + object storage you run on a
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VPS, exposing a clean REST API (and an MCP server) so any agent can own an address, read its mail as
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structured threads, and reply.
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> **Status: private WIP.** This is a personal learning project and a self-hosting alternative in a
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> space that hosted products (e.g. AgentMail) serve well. Keep it private. Don't reference it in any
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> job application, PR, or interview, and shelve it if it would ever conflict with an employer. It was
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> designed only from public API surfaces and public RFCs — nothing proprietary.
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See **[ARCHITECTURE.md](./ARCHITECTURE.md)** for the design.
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## Why
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The hard, valuable part of an agent-mailbox product is not the API — it's the mail plumbing:
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receiving over SMTP/MX, sending with real deliverability (SPF/DKIM/DMARC, IP reputation), parsing
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MIME, threading, and storage at scale. OpenMail is an exercise in building that plumbing properly,
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self-hostable, with an agent-first API on top.
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## Goals
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- **Self-hostable** in one `docker compose up` on a single VPS, scaling to a small fleet later.
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- **Agent-first API** — persistent inboxes, structured threads, `extracted_text` (quoted-history
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stripped), labels, search, drafts, webhooks/WebSocket events.
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- **AgentMail-API-shaped** where reasonable, so existing agent tooling/MCP can point at a self-hosted
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endpoint with minimal change. (Compatibility is a non-goal where it conflicts with a cleaner design.)
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- **Deliverability taken seriously** — DKIM signing, SPF/DMARC guidance, and a pluggable send path
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(self-host SMTP **or** relay through SES/Postmark) because reputation is the real moat.
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## Non-goals (for v1)
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- A hosted multi-tenant SaaS. OpenMail is self-host-first.
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- Beating a mature provider on deliverability out of the box — that takes IP warmup and time.
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- A webmail UI. The product is the API + MCP; humans use their own client or the CLI.
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