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Karti TripathiandClaude Opus 5 36b15ddcaf Rewrite as a Rust, Apache-2.0 workspace

Supersedes the Go + embed-Mox design. The Go tree is removed; its
architecture doc is preserved at docs/archive/ARCHITECTURE-go-embed-mox.md
because its competitive analysis and data model still hold.

Five decisions recorded as ADRs:

  0001  Rust, not Go — accepting ~5,500 lines of protocol code that Mox
        would have given us free, to get the first permissively licensed
        Rust mail server. Costs stated plainly.
  0002  Apache-2.0, not MIT or AGPL — patent grant, trademark, CLA-free
        contribution. Public on GitHub; Gitea stays as the private fallback.
  0003  Stalwart's primitive crates (Apache-2.0/MIT) yes; its AGPL server
        crates never. DANE and MTA-STS sit on the AGPL side of that line,
        which is why we write our own.
  0004  Milestones, reordered: embedded inbound is required at launch.
  0005  Oracle Cloud blocks outbound :25, so direct-to-MX is impossible on
        the launch host. Split delivery is mandatory, not an on-ramp.

Twelve crates in three tiers. Tier 1 (mail-dane, mail-mta-sts, mail-dsn)
is standalone and publishable — no `dane` or `mta-sts` crate exists on
crates.io at all today.

openmail-relay ships the provider table as data, with SES and Oracle from
the start. Oracle's and Resend's SPF includes are deliberately None: a
guessed include turns the DNS check green against a mechanism the provider
does not honour, and mail still fails SPF silently.

cargo check/test/clippy/fmt all green; unsafe_code is forbidden workspace
wide; cargo-deny enforces the licence policy in CI.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JkyvfNJGTshJNE9FtwPLk7

2026-09-02 13:08:05 -07:00

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ADR 0001 — Rust, not Go

Status: Accepted, 2026-09-02. Supersedes the Go + embed-Mox design in ../archive/ARCHITECTURE-go-embed-mox.md.

Context

The archived design chose Go in order to embed Mox (MIT) as a library, getting ~14,000 lines of production-tested mail correctness for free:

message 2,884   dkim 2,051   spf 1,560   smtpclient 2,012   junk 1,302
mtasts    703   dsn    771   dane   516   scram    796   sasl 327   …

The Rust ecosystem does not offer an equivalent. Stalwart is the only production Rust mail server and its server crates are AGPL-3.0-only OR LicenseRef-SEL, which we cannot use under Apache-2.0.

What Rust actually costs

Stalwart Labs publishes its primitives permissively (Apache-2.0 OR MIT), and those cover more than expected: mail-parser (MIME), mail-auth (DKIM1, DKIM2, ARC, SPF, DMARC, ARF, TLS-RPT), mail-builder, mail-send, smtp-proto. hickory-resolver covers DNS and DNSSEC.

What no permissive Rust crate provides, and we therefore write:

	LOC (Mox equivalent)	Rust prior art
DANE	~516	none on crates.io
MTA-STS	~703	none on crates.io
SMTP server session loop	~3,395 (`go-smtp`)	`smtp-proto` parses only
DSN	~771	none
Junk (beyond a toy)	~1,302	`bayespam` has no training persistence
iprev / DNSBL / rate limit	~370	`dnsbl` crate abandoned since 2021

~5,500 lines of adversarial protocol code, versus zero in Go.

Decision

Rust. Accepted with eyes open.

Consequences

Negative, and stated plainly so nobody is surprised later:

v1 is roughly a quarter further out.
DANE and MTA-STS move from battle-tested to ours, and both fail silently: a DANE bug downgrades TLS without erroring; an MTA-STS bug defers mail nobody sees. That tail does not close at ship — it closes after enough strangers' mail has flowed through it.
Mitigation: every outcome in those crates is an explicit enum with no Default and no bool, so a caller cannot accidentally read "no policy" as "verified". See mail_dane::DaneResult.

Positive:

We ship the first permissively licensed DANE and MTA-STS in Rust, and the first permissively licensed Rust mail server.
mail-auth gives us DKIM2 and ARC, which Mox does not have.
One language for the mail engine and the agent layer.

Rejected alternative

Go now, Rust later, with the two crates published early to plant the flag at low cost. Rejected: it puts the strategic position — "the permissive Rust agent mail server" — behind a rewrite that would probably never be scheduled.

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