<!--
SPDX-FileCopyrightText: © 2026 Jeffrey C. Ollie <jeff@ocjtech.us>
SPDX-License-Identifier: MIT
-->

# zsmtp

An SMTP client and server library for Zig (RFC 5321).

Both the client and the server run over plain `std.Io.Reader`/`std.Io.Writer`
pairs, so they are transport-agnostic: wrap a TCP stream for real use, or
fixed in-memory buffers in tests. Requires Zig 0.16.

## Client

```zig
const zsmtp = @import("zsmtp");

var reply_buf: [1024]u8 = undefined;
var client: zsmtp.Client = .init(&stream_reader.interface, &stream_writer.interface, &reply_buf);

_ = try client.greet();                        // read the 220 greeting
_ = try client.hello("my-host.example.com");   // EHLO (HELO fallback), returns extensions
try client.sendMail("me@example.com", &.{"you@example.net"}, message);
try client.quit();
```

Line endings in the message are normalized to CRLF and leading dots are
stuffed automatically. On `error.UnexpectedReply`, `client.last_reply` holds
the server's actual code and text. `mailFrom`/`rcptTo`/`sendMessage` are also
available individually.

Message bodies can also be streamed instead of passed as a slice — from any
reader via `sendMessageReader(&reader)`, or push-style via `data()`, which
returns a writer that dot-stuffs and normalizes line endings as content
flows through it:

```zig
var data_writer = try client.data();
try data_writer.interface.print("Subject: report {d}\r\n\r\n", .{id});
// ... stream as much as needed ...
try data_writer.end(); // terminates the message, reads the verdict
```

When the server advertises CHUNKING (`extensions.chunking`), `bdat` and
`sendMessageChunked` transmit the message with length-framed BDAT chunks
instead of DATA — verbatim, with no dot-stuffing, so content must already
use CRLF line endings.

### Authentication

`hello` reports the server's advertised mechanisms in `extensions.auth`;
`authenticate` picks the best one (PLAIN, then LOGIN, then CRAM-MD5), or use
`authPlain`/`authLogin`/`authCramMd5` directly. PLAIN and LOGIN send
credentials unprotected, so use TLS on real networks. A 535 rejection
surfaces as `error.AuthenticationFailed` with the reply in `last_reply`.

```zig
const extensions = try client.hello("my-host.example.com");
try client.authenticate(extensions, "user", "password");
```

### TLS

`zsmtp.Tls` wraps [ianic/tls.zig](https://github.com/ianic/tls.zig) and
verifies against the system trust store by default (a caller-managed CA
bundle and an insecure mode are also available). The stream reader/writer
handed to it need buffers of at least `zsmtp.Tls.min_buffer_len` bytes, and
`init` must run at the value's final address (the connection holds interior
pointers). The standard library's TLS client is deliberately not used: it
requires the optional TLS 1.3 middlebox-compatibility ChangeCipherSpec
record, which servers like Exim disable.

Implicit TLS (port 465) — handshake first, then speak SMTP:

```zig
var tls: zsmtp.Tls = undefined;
try tls.init(gpa, io, &stream_reader.interface, &stream_writer.interface, .{
    .host = "smtp.example.com",
});
defer tls.deinit(gpa);
var client: zsmtp.Client = .init(tls.reader(), tls.writer(), &reply_buf);
// ... greet, hello, sendMail ...
try client.quit();
try tls.end(); // close_notify, before closing the socket
```

STARTTLS (port 587) — upgrade mid-session, then EHLO again:

```zig
_ = try client.greet();
_ = try client.hello("my-host.example.com"); // check .starttls in the result
try client.starttls();
var tls: zsmtp.Tls = undefined;
try tls.init(gpa, io, &stream_reader.interface, &stream_writer.interface, .{
    .host = "smtp.example.com",
});
client.setTransport(tls.reader(), tls.writer());
_ = try client.hello("my-host.example.com"); // server state was reset
```

## Server

```zig
var session: zsmtp.Server = .init(&stream_reader.interface, &stream_writer.interface, .{
    .context = &my_state,
    .vtable = &.{
        .authenticate = onAuth, // optional; enables AUTH PLAIN and LOGIN
        .rcptTo = onRcptTo,     // optional; accept/reject each recipient
        .message = onMessage,   // required; receives envelope + message data
    },
}, .{ .hostname = "mx.example.com" });
try session.run(gpa);
```

With an `authenticate` callback the session advertises and accepts AUTH
PLAIN and AUTH LOGIN (RFC 4954); setting `Options.require_auth` rejects MAIL
with 530 until the client has authenticated.

Instead of `message` (which collects the whole body in memory, bounded by
`max_message_size`), a handler can set `messageReader` to stream it: the
callback receives an `Io.Reader` yielding the unstuffed message content,
and anything left unread is drained by the session.

`run` serves one connection until QUIT or disconnect, enforcing command
sequencing, recipient and message-size limits, and un-stuffing message data.
Messages may also arrive via BDAT chunks (CHUNKING is advertised); both
the collecting and streaming handler paths receive the reassembled content.
MAIL parameters are validated: `SIZE=` (RFC 1870) is rejected early with 552
when it exceeds `max_message_size`, `BODY=7BIT`/`BODY=8BITMIME` (RFC 6152)
are accepted, and unrecognized parameters get 555; the declared size and
body type reach the handler via `Envelope`. Listening, accepting, and
concurrency are up to the caller.

To advertise and accept STARTTLS (TLS 1.3, via
[ianic/tls.zig](https://github.com/ianic/tls.zig)), pass a certificate key
pair; the stream buffers must then be at least `zsmtp.tls.input_buffer_len` /
`zsmtp.tls.output_buffer_len` bytes, since the handshake runs over them:

```zig
var auth: zsmtp.tls.config.CertKeyPair =
    try .fromFilePath(gpa, io, .cwd(), "cert.pem", "key.pem");
defer auth.deinit(gpa);

var session: zsmtp.Server = .init(&stream_reader.interface, &stream_writer.interface, handler, .{
    .hostname = "mx.example.com",
    .tls = .{ .io = io, .auth = &auth },
});
try session.run(gpa);
```

On STARTTLS the session answers 220, performs the server handshake, swaps
its transport to the encrypted connection, and resets state per RFC 3207 (the
client must EHLO again). With `.mode = .implicit` the handshake instead runs
before the greeting (SMTPS, port 465 style):

```zig
var session: zsmtp.Server = .init(&stream_reader.interface, &stream_writer.interface, handler, .{
    .hostname = "mx.example.com",
    .tls = .{ .io = io, .auth = &auth, .mode = .implicit },
});
```

## Demo CLI

```sh
zig build

# Debug server that prints received messages to stdout
# (with a cert/key pair it advertises and accepts STARTTLS):
./zig-out/bin/zsmtp serve 2525
./zig-out/bin/zsmtp serve --tls-cert cert.pem --tls-key key.pem 2525
./zig-out/bin/zsmtp serve --tls-cert cert.pem --tls-key key.pem --implicit-tls 2465

# Send a message read from stdin:
printf 'Subject: hi\r\n\r\nhello\r\n' | \
    ./zig-out/bin/zsmtp send 127.0.0.1 2525 me@example.com you@example.net

# Same, over implicit TLS or STARTTLS (--insecure skips cert verification):
zsmtp send --tls smtp.example.com 465 me@example.com you@example.net
zsmtp send --starttls smtp.example.com 587 me@example.com you@example.net
```

## Status

TLS is supported on both sides via
[ianic/tls.zig](https://github.com/ianic/tls.zig): the client does implicit
TLS and STARTTLS via `zsmtp.Tls`, and the server accepts both STARTTLS and
implicit TLS (TLS 1.3 only). AUTH covers PLAIN, LOGIN, and CRAM-MD5 on the
client and PLAIN and LOGIN on the server. Message bodies can be streamed on
both sides, and the server validates MAIL parameters (SIZE=, BODY=).

## Standards

- [RFC 5321](https://datatracker.ietf.org/doc/html/rfc5321) — Simple Mail
  Transfer Protocol: the command/reply protocol, multiline replies,
  dot-stuffing, reply classes, and ESMTP parameter syntax (client and
  server).
- [RFC 1870](https://datatracker.ietf.org/doc/html/rfc1870) — SIZE:
  advertised and enforced by the server (oversize declarations are rejected
  with 552 before DATA); parsed from EHLO by the client.
- [RFC 6152](https://datatracker.ietf.org/doc/html/rfc6152) — 8BITMIME:
  advertised by the server and `BODY=` validated; parsed by the client.
- [RFC 3030](https://datatracker.ietf.org/doc/html/rfc3030) — CHUNKING
  (BDAT): client and server, with length-based framing and no dot-stuffing;
  the companion BINARYMIME extension is not implemented (`BODY=BINARYMIME`
  is rejected).
- [RFC 2920](https://datatracker.ietf.org/doc/html/rfc2920) — PIPELINING:
  advertised by the server, whose strictly sequential command loop handles
  pipelined clients naturally; parsed by the client.
- [RFC 3207](https://datatracker.ietf.org/doc/html/rfc3207) — STARTTLS:
  client and server, including the mandatory post-handshake state reset.
- [RFC 8314](https://datatracker.ietf.org/doc/html/rfc8314) — implicit TLS
  (SMTPS): client (`Tls` before any SMTP traffic) and server
  (`.mode = .implicit`).
- [RFC 4954](https://datatracker.ietf.org/doc/html/rfc4954) — AUTH: client
  and server, including initial responses and `*` cancellation.
- [RFC 4616](https://datatracker.ietf.org/doc/html/rfc4616) — the PLAIN
  SASL mechanism (client and server).
- [RFC 2195](https://datatracker.ietf.org/doc/html/rfc2195) — CRAM-MD5
  (client only; the server would need plaintext-equivalent credentials).
- [draft-murchison-sasl-login](https://datatracker.ietf.org/doc/html/draft-murchison-sasl-login-00)
  — the de-facto AUTH LOGIN mechanism (client and server).
- [RFC 3463](https://datatracker.ietf.org/doc/html/rfc3463) /
  [RFC 2034](https://datatracker.ietf.org/doc/html/rfc2034) — enhanced
  status codes: carried in every server reply and advertised via
  ENHANCEDSTATUSCODES; detected by the client.
- [RFC 6531](https://datatracker.ietf.org/doc/html/rfc6531) — SMTPUTF8:
  client (`mailFromUtf8`) and server (advertised; non-ASCII addresses
  require the parameter and must be valid UTF-8, rejected with 553 5.6.7
  per [RFC 6533](https://datatracker.ietf.org/doc/html/rfc6533) otherwise;
  the flag reaches handlers via `Envelope.smtputf8`).

TLS itself (TLS 1.3, [RFC 8446](https://datatracker.ietf.org/doc/html/rfc8446))
is provided by [ianic/tls.zig](https://github.com/ianic/tls.zig).

## Tests

```sh
zig build test
zig build test --fuzz   # run the fuzz tests under the fuzzer (endless)
```

The fuzz tests cover parser crash-safety (`Command.parse`, `Reply.read`),
whole-session robustness against arbitrary bytes on both the client and
server side, and two differential properties: the streaming `DataWriter`
must produce byte-identical output to the slice-based `writeStuffed` under
fuzzer-chosen chunk boundaries, and the collecting and streaming server
DATA paths must yield identical message content.

### Protocol torture testing with exim's test client

Exim's scriptable SMTP test client (`test/src/client.c` in the exim
source) sends raw protocol lines and asserts reply prefixes. The exim
source is declared as a *lazy* Zig dependency, fetched only on demand:

```sh
zig build -Dexim-client        # fetches exim, installs zig-out/bin/exim-client
./zig-out/bin/zsmtp serve 2525 &
./zig-out/bin/exim-client 127.0.0.1 2525 < test/protocol-torture.script
```

`test/protocol-torture.script` is a 28-reply dialogue distilled from
exim's own test suite (syntax errors, sequencing violations, parameter
validation, dot-stuffing); the same dialogue is asserted byte-for-byte
as a unit test in `Server.zig`.

The library is MIT-licensed; the small amount of test-only material adapted
from exim's test suite (the torture script and the gauntlet unit test's
dialogue) is GPL-2.0-or-later, marked with SPDX snippet tags and REUSE.toml
annotations.

Note: Zig 0.16.0's fuzz *driver* is broken out of the box (its bundled
test runner fails to compile in fuzz mode, and the coverage server panics
on a test binary with no fuzz tests); both are fixed on Zig master. Until
then, fuzzing needs a patched copy of the standard library via
`zig build --zig-lib-dir <patched-lib> test --fuzz`. The fuzz tests
themselves also run once per invocation as part of the normal
`zig build test` suite.

Interoperability against third-party implementations is covered by a NixOS
VM test (`nix/interop-test.nix`): the zsmtp client delivers mail to Postfix
and Exim over plaintext, STARTTLS, and implicit TLS against each, and swaks
delivers to the zsmtp server over plaintext and STARTTLS.

```sh
nix build .#zsmtp                        # build the package
nix build .#checks.x86_64-linux.interop  # run the VM interop test
```