# OpenMail — Architecture

Agent-native, self-hosted mail server. **One Go binary** that embeds [Mox](https://github.com/mjl-/mox)'s
(MIT) mail internals for the correctness-critical 95% — DKIM, SPF/DMARC, DANE + MTA-STS delivery,
real-world MIME, spam filtering — and owns a **native, agent-shaped data model** (Postgres + object
store) with a REST API and MCP server on top. This document is the design of record. It favors an
MVP that actually receives and sends real mail over a perfect feature set.

## 0. The decision: embed Mox, don't rebuild and don't just wrap it

An agent-mailbox API is ~5% of the work. The other 95% is mail infrastructure (inbound :25 hardening,
outbound deliverability, MIME, threading, quote-stripping, anti-abuse). Three ways to get that 95%:

1. **Build from scratch** — re-implement DKIM/SPF/DMARC/DANE/MTA-STS/MIME ourselves. Correct but slow,
   and we'd do it worse than a production server at first.
2. **Sidecar an existing server** (run Mox as a separate process, read it over IMAP). Fast, but we
   don't own the data model — threads/labels/`extracted_text` would be re-derived from IMAP, and we
   ship two coupled processes.
3. **Embed Mox's packages** into our own binary and own the storage + API. ← **chosen.**

Why (3) is viable: **Mox has no `internal/` directory** — every package is importable — and the
README states *"most non-server Go packages are written to be reusable."* The real boundary is not
visibility, it's **storage coupling**: Mox's *server + store* packages are welded to its embedded
`bstore`/`bbolt` store and global config; its *protocol/crypto/parsing* packages are stateless and
clean. We embed the latter and replace the former with our own Postgres-backed server.

### What we embed from Mox vs. what we own

| Concern | Source | Notes |
|---|---|---|
| MIME parsing | **mox `message`** | Battle-tested on real, broken senders. Better than enmime for our needs. |
| DKIM sign + verify | **mox `dkim`** | |
| SPF / DMARC | **mox `spf`, `dmarc`** | Inbound auth verification + policy. |
| Secure outbound delivery | **mox `smtpclient`, `dane`, `mtasts`, `dns`** | The deliverability moat — DANE/MTA-STS MX delivery, mostly solved by reusing this. |
| Bounces (DSN) | **mox `dsn`** | Parse/generate delivery status notifications. |
| Anti-abuse | **mox `iprev`, `dnsbl`, `ratelimit`** | rDNS check, blocklists, rate limiting on :25. |
| Spam filtering | **mox `junk`** | Built-in Bayesian filter; per-inbox train/score. |
| SMTP auth | **mox `sasl`, `scram`** | For the submission (send) path. |
| — | — | — |
| Inbound SMTP **server loop** | **`emersion/go-smtp`** | Clean, embeddable listener/session. We do NOT import mox `smtpserver` (store-coupled). |
| Storage | **OpenMail (Postgres + S3)** | Native agent data model. NOT mox `store`/bbolt. |
| API, MCP, threading, extraction, webhooks, auth, multi-tenancy | **OpenMail** | The agent-native layer — our differentiation. |

> **Boundary rule:** never import `smtpserver`, `imapserver`, `queue`, `store`, or `mox-` (config) —
> they assume Mox's storage/config. Importing them is the line between "embed as a library" and
> "fork Mox." We stay on the library side.

## 1. Component overview

```
                         ┌──────────────────────────────────────────────────────┐
   Internet senders ───▶ │  smtpd  (go-smtp on :25)                              │
   (MX → this VPS)       │   iprev/dnsbl/ratelimit → SPF/DKIM/DMARC verify       │
                         │   → mox/message parse → mox/junk score → enqueue      │   ← mox pkgs
                         └───────────────┬──────────────────────────────────────┘
                                         │ internal queue (Postgres LISTEN/NOTIFY)
                         ┌───────────────▼──────────────────────────────────────┐
                         │  core  (Go)                                            │
   Agents / SDKs ──REST──▶   • inbox/message/thread/draft services               │
   MCP clients ───MCP──▶ │   • threading, extracted_text, labels, FTS search     │
                         │   • auth (API keys, pods), webhooks/WS                 │
                         └──────┬───────────────────────────────┬────────────────┘
                                │                                │
                   ┌────────────▼─────┐            ┌─────────────▼──────────────┐
                   │ Postgres         │            │ Object store (S3/MinIO)    │
                   │ metadata + FTS   │            │ raw .eml + attachments     │
                   └──────────────────┘            └────────────────────────────┘
                                         ▲
                         ┌───────────────┴──────────────────────────────────────┐
   Outbound to world ◀── │  sender  (mox/dkim sign → backend)                    │
                         │   self-host: mox/smtpclient + dane + mtasts (MX)      │   ← mox pkgs
                         │   relay:     SES / Postmark / Resend API              │
                         └──────────────────────────────────────────────────────┘
```

All roles (`smtpd`, `core` API, `sender`, `mcp`) ship in one binary with subcommands; deploy together
or split later. Caddy terminates TLS for the HTTP API and auto-manages certs.

## 2. Tech stack

- **Language:** Go — single static binary, and the whole mature self-hosted-mail ecosystem (Mox,
  go-smtp, go-message) is Go, keeping us MIT-compatible. Rust (Stalwart) is AGPL → ruled out.
- **HTTP:** `chi` router; `sqlc` + `pgx` for typed Postgres access.
- **Inbound server:** `emersion/go-smtp` listener → handed to mox packages for verify/parse/score.
- **Mail correctness/delivery:** mox `message`, `dkim`, `spf`, `dmarc`, `dane`, `mtasts`, `dns`,
  `smtpclient`, `dsn`, `junk`, `iprev`, `dnsbl`, `ratelimit`, `sasl`, `scram` (see §0 table).
- **Quote stripping:** a `talon`/`EmailReplyParser`-style heuristic pass for `extracted_text`.
- **Storage:** Postgres (metadata + `tsvector` FTS); S3-compatible object store (MinIO self-host, or
  S3) for raw `.eml` and attachment blobs.
- **Events:** Postgres `LISTEN/NOTIFY` for the internal inbound queue and to fan out webhooks + a
  WebSocket stream. (Swap to NATS only if volume warrants.)
- **Deploy:** `docker compose` — `caddy`, `openmail` (one image, multiple commands), `postgres`,
  `minio`. MX + SPF/DKIM/DMARC/DANE/MTA-STS DNS documented in `deploy/`.

## 3. Data model (Postgres)

Native and agent-shaped (not Mox's per-account bbolt index). Resource shapes mirror what agent tooling
expects so SDKs/MCP map cleanly.

```sql
pods          (id, name, created_at)                       -- tenant isolation
inboxes       (id, pod_id, address UNIQUE, display_name, dkim_selector, metadata jsonb, created_at, updated_at)
threads       (id, inbox_id, subject, last_message_id, message_count, labels text[], updated_at, created_at)
messages      (id, inbox_id, thread_id, message_id_hdr, in_reply_to, references text[],
               from_addr, to_addrs text[], cc text[], bcc text[], subject,
               preview, text, html, extracted_text, extracted_html,
               raw_object_key,            -- pointer to raw .eml in object store
               spf, dkim, dmarc,          -- inbound auth verdicts (from mox pkgs)
               junk_score, labels text[], size_bytes, headers jsonb, ts tsvector,  -- FTS
               created_at, updated_at)
attachments   (id, message_id, filename, content_type, size_bytes, object_key, inline bool, content_id)
drafts        (id, inbox_id, thread_id, to_addrs text[], cc, bcc, subject, text, html, send_at, client_id)
api_keys      (id, pod_id, hash, scopes text[], created_at)   -- bearer tokens; store only the hash
webhooks      (id, pod_id, url, event_types text[], secret)   -- HMAC-signed deliveries
outbox        (id, message_id, status, attempts, last_error, next_attempt_at)  -- send queue + retries
events        (id, inbox_id, type, payload jsonb, created_at) -- audit + WS replay
domains       (id, pod_id, name, dkim_privkey_ref, verified bool)  -- per-domain DKIM keys + DNS state
```

Indexes: `inboxes(address)`, `messages(thread_id)`, `messages(inbox_id, created_at desc)`,
GIN on `messages.ts` (search) and `*.labels`. Raw MIME + attachments live in object storage; rows
hold pointers.

## 4. Request / data flows

### Inbound (receive)
1. `go-smtp` accepts on :25; anti-abuse gate via mox `iprev`/`dnsbl`/`ratelimit` + size cap; raw `.eml`
   written to object store.
2. mox `spf`/`dkim`/`dmarc` verify the message; verdicts recorded. mox `message` builds the MIME tree
   → `text`/`html`, attachments, headers. mox `junk` scores spam. Quote-stripper computes
   `extracted_text` + `preview`.
3. Threading resolves `thread_id` from `In-Reply-To`/`References`, falling back to normalized-subject
   + participants within a window.
4. Row inserted; `LISTEN/NOTIFY` fires → `message.received` webhook + WebSocket event.

### Outbound (send / reply)
1. API builds the RFC 5322 message (sets `In-Reply-To`/`References` on replies); persists a `messages`
   row + `outbox` entry.
2. `sender` mox-`dkim`-signs, then dispatches via the configured backend:
   - **self-host:** mox `smtpclient` + `dane` + `mtasts` for authenticated, secure MX delivery.
   - **relay:** SES/Postmark/Resend API.
3. Bounces/complaints (mox `dsn` parse, or relay webhook) update `outbox.status` and emit events.

### API surface (v1 — AgentMail-shaped)
```
POST   /v0/inboxes                         create inbox
GET    /v0/inboxes  /  /v0/inboxes/{id}    list / get
POST   /v0/inboxes/{id}/messages/send      send
GET    /v0/inboxes/{id}/messages           list (limit, page_token, labels)
GET    /v0/inboxes/{id}/messages/{id}      get
POST   /v0/inboxes/{id}/messages/{id}/reply
GET    /v0/inboxes/{id}/threads  /  /{id}  list / get
…drafts, webhooks, search…
```
Bearer auth (`Authorization: Bearer`). Keeping paths/shapes close to AgentMail's public v0 lets an
existing client (or MCP server) target a self-hosted OpenMail by base-URL swap. Compatibility yields
to a cleaner native shape where they conflict.

### MCP server
A thin MCP front-end over `core`: tools `create_inbox`, `list_messages`, `get_thread`, `send_message`,
`reply`, `search` — so an agent drives its own mailbox directly.

## 5. Deliverability plan (the moat — and why embedding Mox helps most here)

- **Self-host send is achievable from day one for the protocol parts** because mox `smtpclient` +
  `dane` + `mtasts` + `dkim` already implement authenticated, DANE/MTA-STS-secured MX delivery. What
  remains is *operational*, not code: dedicated IP with PTR/rDNS, SPF (`v=spf1 ip4:… -all`), published
  DKIM key, DMARC (`p=quarantine`→`p=reject`), gradual IP warmup, FBL enrollment, suppression lists.
- **Relay backend** (SES/Postmark/Resend) ships alongside for instant inbox placement while a
  self-host IP warms up — chosen per-domain.
- **Inbound auth:** SPF/DKIM/DMARC verified on receipt (mox pkgs); verdicts exposed in message
  metadata; optional reject/quarantine policy per inbox.

## 6. MVP milestones

1. **Core API + storage** — inboxes/messages/threads/drafts CRUD on Postgres + MinIO; bearer auth;
   ingest endpoint to seed without real mail. *Proves the data model + API.*
2. **Inbound** — `go-smtp` on :25 + mox verify/parse/junk + threading + `message.received` webhook.
   MX a test domain at the VPS. *Proves receive.*
3. **Outbound via relay** — `send`/`reply`, mox-DKIM-signed, through a relay. *Proves the loop: an
   agent receives and replies.*
4. **MCP server** — agent owns and operates an inbox end-to-end.
5. **Self-host SMTP send** — wire mox `smtpclient`/`dane`/`mtasts`; the deliverability/warmup long tail.

## 7. Repo layout (planned)

```
cmd/openmail/         # single binary: `serve`, `smtpd`, `sender`, `mcp` subcommands
internal/core/        # inbox/message/thread/draft services
internal/smtp/        # go-smtp inbound server; calls mox verify/parse/junk + threading
internal/sender/      # Sender interface: smtp (mox smtpclient+dane+mtasts) & relay backends; mox dkim
internal/store/             # pgx/sqlc queries, object-store client, embedded migrations
internal/store/migrations/  # SQL migrations (go:embed'd into the binary)
internal/api/         # chi HTTP handlers (v0 surface)
internal/mcp/         # MCP server over core
deploy/               # docker-compose.yml, Caddyfile, DNS (MX/SPF/DKIM/DMARC/DANE/MTA-STS) notes
spike/                # throwaway feasibility spikes (mimecheck: mox standalone parse — PASSED)
```

## 8. Licensing & attribution

OpenMail is **MIT** (see `LICENSE`). Every dependency is permissive: Mox (MIT), `emersion/go-smtp`
and `go-message` (MIT), `pgx`/`chi` (MIT/BSD). We import Mox as a Go module (no vendored source in
this repo), but credit it prominently in the README; if we ever vendor Mox source we must retain its
MIT notice. **No GPL/AGPL** code anywhere — Maddy (GPL-3.0) and Stalwart (AGPL) are deliberately
avoided so the project stays MIT.

## 9. Open questions

- **mox `message` ⇄ go-smtp seam:** confirm mox's parser consumes a raw reader cleanly outside mox's
  store (spike in milestone 2). If any Tier-1 package drags in `mox-`/`store` transitively, isolate
  behind a thin adapter or vendor just that file.
- How far to chase AgentMail API compatibility vs. a cleaner native shape.
- Search: Postgres FTS is enough for v1; revisit (Meilisearch/Typesense) only if needed.
- Multi-tenancy depth — `pods` now, full RBAC later.
- Whether to expose an optional IMAP read path (via mox `imapserver` over our store would mean forking
  it — more likely a thin standalone IMAP front-end later, kept out of v1).
```