An SMTP client and server library for Zig implementing RFC 5321.
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1<!--
2SPDX-FileCopyrightText: © 2026 Jeffrey C. Ollie <jeff@ocjtech.us>
3SPDX-License-Identifier: MIT
4-->
5
6# zsmtp
7
8An SMTP client and server library for Zig (RFC 5321).
9
10Both the client and the server run over plain `std.Io.Reader`/`std.Io.Writer`
11pairs, so they are transport-agnostic: wrap a TCP stream for real use, or
12fixed in-memory buffers in tests. Requires Zig 0.16.
13
14## Where this lives
15
16The canonical repository is on Forgejo, with mirrors on Tangled and Radicle:
17
18- <https://git.jcollie.dev/jeff/zsmtp> — issues and pull requests
19- <https://tangled.org/jcollie.dev/zsmtp>
20
21```sh
22git clone https://git.jcollie.dev/jeff/zsmtp.git
23```
24
25On [Radicle](https://radicle.xyz/), the peer-to-peer forge, the repository is
26`rad:z3ZKHgoDKEue8FT7sV6fHZdtjxRx1`, which is the only name it has there — a
27Radicle repository is found by its ID and nothing else — so seeding or cloning
28it goes:
29
30```sh
31rad clone rad:z3ZKHgoDKEue8FT7sV6fHZdtjxRx1
32```
33
34Cloning also seeds the repository, which helps keep it available on the
35network.
36
37The API documentation is generated from the doc comments and published at
38<https://jeff.jcollie.page/zsmtp/>; `zig build docs` builds it locally and
39`zig build docs-serve` serves it for reading.
40
41## Client
42
43```zig
44const zsmtp = @import("zsmtp");
45
46var reply_buf: [1024]u8 = undefined;
47var client: zsmtp.Client = .init(&stream_reader.interface, &stream_writer.interface, &reply_buf);
48
49_ = try client.greet(); // read the 220 greeting
50_ = try client.hello("my-host.example.com"); // EHLO (HELO fallback), returns extensions
51try client.sendMail("me@example.com", &.{"you@example.net"}, message);
52try client.quit();
53```
54
55Line endings in the message are normalized to CRLF and leading dots are
56stuffed automatically. On `error.UnexpectedReply`, `client.last_reply` holds
57the server's actual code and text. `mailFrom`/`rcptTo`/`sendMessage` are also
58available individually.
59
60Addresses and the EHLO domain are checked before they are written: a value
61containing CR, LF or NUL is rejected with `error.UnsafeArgument` rather than
62sent, since it would otherwise end the command line early and let the rest of
63it be read as further SMTP commands. The check is `protocol.isSafeArgument`,
64and it is framing only — it does not claim the address is a well-formed
65mailbox.
66
67Message bodies can also be streamed instead of passed as a slice — from any
68reader via `sendMessageReader(&reader)`, or push-style via `data()`, which
69returns a writer that dot-stuffs and normalizes line endings as content
70flows through it:
71
72```zig
73var data_writer = try client.data();
74try data_writer.interface.print("Subject: report {d}\r\n\r\n", .{id});
75// ... stream as much as needed ...
76try data_writer.end(); // terminates the message, reads the verdict
77```
78
79`envelope` sends MAIL FROM and every RCPT TO at once and reads all their
80replies, which against a server advertising PIPELINING
81([RFC 2920](https://datatracker.ietf.org/doc/html/rfc2920)) turns an envelope
82of *n* recipients from *n*+1 round trips into one. `hello` sets
83`client.pipelining` from the EHLO response and `envelope` falls back to
84waiting for each reply when it is false, so the result is the same either
85way:
86
87```zig
88var codes: [3]u16 = undefined;
89const accepted = try client.envelope(from, recipients, &codes, .{});
90// codes[i] is the RCPT reply code for recipients[i].
91```
92
93A refused recipient is not an error — with several of them the caller is the
94one who can say whether what remains is worth sending — so compare `accepted`
95against `recipients.len`. `sendMail` makes that decision the strict way: if
96any recipient was refused it sends RSET and returns `error.UnexpectedReply`
97without delivering to the others.
98
99DATA is deliberately left out of the group, though RFC 2920 allows it as the
100last command of one. Once a server has answered DATA with 354 the transaction
101is committed, and the only ways out are to send the message or to send an
102empty one to whichever recipients were accepted; stopping the group before
103DATA keeps that choice with the caller, and costs one round trip out of the
104*n*+1 saved.
105
106`mail` and `rcpt` are the parameterized forms of `mailFrom` and `rcptTo`,
107carrying the ESMTP parameters the server advertised — today SMTPUTF8 and the
108DSN set of [RFC 3461](https://datatracker.ietf.org/doc/html/rfc3461):
109
110```zig
111try client.mail("me@example.com", .{ .ret = .hdrs, .envid = "batch 7" });
112try client.rcpt("bob@example.net", .{
113 .notify = .{ .on = .{ .failure = true, .delay = true } },
114 .orcpt = .{ .addr_type = "rfc822", .address = "team@example.net" },
115});
116```
117
118`ENVID` and the `ORCPT` address are xtext-encoded on the way out, so any
119bytes are safe to pass; the length limits RFC 3461 puts on the encoded form
120(100 and 500 characters) are checked and surface as
121`error.ArgumentTooLong`. Check `extensions.dsn` first — a conforming server
122answers an unrecognized parameter with 555.
123
124Setting `client.mode = .lmtp` before `hello` speaks LMTP: `LHLO` goes out in
125place of `EHLO`, and the end of a message brings back one verdict per
126accepted recipient, in the order the RCPT commands were issued. `endResults`
127is how to read them:
128
129```zig
130var data_writer = try client.data();
131try data_writer.interface.writeAll(message);
132var verdicts = try data_writer.endResults();
133while (try verdicts.next()) |reply| {
134 // verdicts.index counts the recipients as they are answered.
135 std.log.info("{s}: {d} {s}", .{ recipients[verdicts.index - 1], reply.code, reply.text });
136}
137```
138
139Every verdict must be read before the session is used again, or the next
140command is answered by a leftover reply. The simpler `end` reads them all
141and reports `error.RecipientRejected` if any was a refusal — without saying
142which, because the replies share one buffer and reading the next overwrites
143the previous.
144
145When the server advertises CHUNKING (`extensions.chunking`), `bdat` and
146`sendMessageChunked` transmit the message with length-framed BDAT chunks
147instead of DATA — verbatim, with no dot-stuffing, so text content must
148already use CRLF line endings.
149
150That framing is also what makes binary content possible.
151`mail(from, .{ .body = .binary_mime })` declares it
152([RFC 3030](https://datatracker.ietf.org/doc/html/rfc3030), needs
153`extensions.binary_mime`), after which the message may hold any octets at
154all — NULs, bare CR, a line that is nothing but a dot — and `data` refuses
155to open a DATA phase for it with `error.BinaryRequiresChunking`, which is
156the 503 the server would have sent, made one round trip earlier. RFC 3030
157is absolute that binary must not be sent to a server that did not advertise
158it, so check the capability first.
159
160### Authentication
161
162The mechanisms themselves live in
163[zig-sasl](https://git.jcollie.dev/jeff/zig-sasl), re-exported here as
164`zsmtp.sasl`, because nothing about PLAIN or CRAM-MD5 or XOAUTH2 is specific
165to SMTP — POP3 and IMAP want the same ones, and one implementation of each is
166better than three. What is specific to SMTP is `authenticate`: the `AUTH`
167command, the 334 challenges, the `*` that cancels, and the 235 that ends it.
168
169`hello` reports the server's advertised mechanism names in `extensions.auth`,
170exactly as it sent them, for `sasl.Client.selectFromList`:
171
172```zig
173var plain: zsmtp.sasl.Plain = .init("user", "password");
174var cram: zsmtp.sasl.CramMd5 = .init("user", "password");
175
176const extensions = try client.hello("my-host.example.com");
177const mechanism = zsmtp.sasl.Client.selectFromList(
178 &.{ plain.client(), cram.client() }, // in order of preference
179 extensions.auth,
180 client.security == .encrypted,
181) orelse return error.NoSupportedMechanism;
182try client.authenticate(mechanism);
183```
184
185A 535 rejection surfaces as `error.AuthenticationFailed` with the reply in
186`last_reply`.
187
188PLAIN, LOGIN and the OAuth mechanisms put a credential on the wire that an
189eavesdropper could reuse — base64 is not encryption, and a bearer token is
190worth more than a password because it authorizes elsewhere too. The client
191refuses those unless `client.security` is `.encrypted`, returning
192`error.InsecureTransport` before anything is sent, and `selectFromList`
193skips them for the same reason: on a plaintext session the preference order
194above falls through PLAIN to CRAM-MD5, which sends a proof rather than the
195secret.
196
197The library is handed a reader and a writer and cannot see what is underneath
198them, so it assumes the worst: `setTransport` records the answer for a
199STARTTLS upgrade, and a session speaking TLS from the first byte sets
200`client.security = .encrypted` itself. For a connection protected by
201something the library cannot see — a unix socket, an SSH tunnel, a loopback
202test — `client.allow_cleartext_auth = true` permits them without claiming the
203transport is encrypted.
204
205One error is worth knowing about even if it never fires for PLAIN:
206`error.ServerNotAuthenticated` means the server reported success while the
207mechanism had not finished proving what it set out to prove. For a one-way
208mechanism that cannot happen. For SCRAM (via
209[zig-scram](https://git.jcollie.dev/jeff/zig-scram)'s `scram-sasl` module) it
210means the server never produced its own signature — which is what something
211in the middle, holding no verifier, would do.
212
213### TLS
214
215`zsmtp.Tls` wraps [ianic/tls.zig](https://github.com/ianic/tls.zig) and
216verifies against the system trust store by default (a caller-managed CA
217bundle and an insecure mode are also available). The stream reader/writer
218handed to it need buffers of at least `zsmtp.Tls.min_buffer_len` bytes, and
219`init` must run at the value's final address (the connection holds interior
220pointers). The standard library's TLS client is deliberately not used: it
221requires the optional TLS 1.3 middlebox-compatibility ChangeCipherSpec
222record, which servers like Exim disable.
223
224Implicit TLS (port 465) — handshake first, then speak SMTP:
225
226```zig
227var tls: zsmtp.Tls = undefined;
228try tls.init(io, gpa, &stream_reader.interface, &stream_writer.interface, .{
229 .host = "smtp.example.com",
230});
231defer tls.deinit(gpa);
232var client: zsmtp.Client = .init(tls.reader(), tls.writer(), &reply_buf);
233client.security = .encrypted; // the transport is TLS; `init` cannot tell
234// ... greet, hello, sendMail ...
235try client.quit();
236try tls.end(); // close_notify, before closing the socket
237```
238
239STARTTLS (port 587) — upgrade mid-session, then EHLO again:
240
241```zig
242_ = try client.greet();
243_ = try client.hello("my-host.example.com"); // check .starttls in the result
244try client.starttls();
245var tls: zsmtp.Tls = undefined;
246try tls.init(io, gpa, &stream_reader.interface, &stream_writer.interface, .{
247 .host = "smtp.example.com",
248});
249client.setTransport(tls.reader(), tls.writer(), .encrypted);
250_ = try client.hello("my-host.example.com"); // server state was reset
251```
252
253## Server
254
255```zig
256var session: zsmtp.Server = .init(&stream_reader.interface, &stream_writer.interface, .{
257 .context = &my_state,
258 .vtable = &.{
259 .authenticate = onAuth, // optional; enables AUTH PLAIN and LOGIN
260 .rcptTo = onRcptTo, // optional; accept/reject each Recipient
261 .message = onMessage, // required; receives envelope + message data
262 },
263}, .{ .hostname = "mx.example.com" });
264try session.run(gpa);
265```
266
267With an `authenticate` callback the session advertises and accepts AUTH
268PLAIN and AUTH LOGIN (RFC 4954); setting `Options.require_auth` rejects MAIL
269with 530 until the client has authenticated.
270
271Instead of `message` (which collects the whole body in memory, bounded by
272`max_message_size`), a handler can set `messageReader` to stream it: the
273callback receives an `Io.Reader` yielding the unstuffed message content,
274and anything left unread is drained by the session.
275
276`run` serves one connection until QUIT or disconnect, enforcing command
277sequencing, recipient and message-size limits, and un-stuffing message data.
278Messages may also arrive via BDAT chunks (CHUNKING is advertised); both
279the collecting and streaming handler paths receive the reassembled content.
280MAIL parameters are validated: `SIZE=` (RFC 1870) is rejected early with 552
281when it exceeds `max_message_size`, `BODY=7BIT`/`BODY=8BITMIME` (RFC 6152)
282are accepted, and unrecognized parameters get 555; the declared size and
283body type reach the handler via `Envelope`. Listening, accepting, and
284concurrency are up to the caller.
285
286The server holds back the replies that RFC 2920 §3.2 permits — RSET, MAIL
287FROM and RCPT TO — so that a pipelined group is answered in one write, and
288sends everything pending the moment its input is empty. The condition is what
289makes that safe rather than a deadlock: a reply is only ever held while there
290is another command already waiting to be answered.
291
292Setting `Options.protocol = .lmtp` makes the session speak LMTP
293([RFC 2033](https://datatracker.ietf.org/doc/html/rfc2033)) instead: `LHLO`
294greets and `HELO`/`EHLO` are refused with 500, and the end of a message
295draws one reply per accepted recipient rather than one for the message —
296including a second reply for a recipient named twice. The `recipientResult`
297callback supplies each verdict:
298
299```zig
300fn onRecipientResult(ctx: ?*anyopaque, envelope: zsmtp.Server.Envelope, index: usize) zsmtp.Server.Decision {
301 return if (mailboxIsFull(envelope.recipients[index].address))
302 .{ .reject = .{ .code = 452, .text = "4.2.2 Mailbox full" } }
303 else
304 .accept;
305}
306```
307
308Without it every recipient is told the same thing, which is correct but
309gains nothing over SMTP. A message the handler rejected outright is reported
310as that rejection for each recipient, since it failed for all of them. LMTP
311is meant for the hop between a queueing MTA and whatever writes to mailboxes;
312RFC 2033 §5 forbids it on TCP port 25 and advises against wide-area use.
313
314BINARYMIME ([RFC 3030](https://datatracker.ietf.org/doc/html/rfc3030)) is
315advertised alongside CHUNKING, which the RFC requires of anything offering
316it. `BODY=BINARYMIME` arrives as `Envelope.body`, DATA for such a message is
317refused with 503, and the content reaches the handler exactly as it was
318sent — the BDAT path copies octets and has no line structure to normalize.
319
320DSN ([RFC 3461](https://datatracker.ietf.org/doc/html/rfc3461)) is
321advertised. `RET=` and `ENVID=` on MAIL arrive as `Envelope.ret` and
322`Envelope.envid`, and `NOTIFY=` and `ORCPT=` on RCPT arrive as
323`Recipient.notify` and `Recipient.orcpt` — at the `rcptTo` callback, which
324receives the whole `Recipient`, and again on the `Envelope` afterwards. The
325xtext values are decoded, the length limits enforced, and a malformed value
326answered with 501. Like everything else handed to a callback, those slices
327live only for the duration of the call; keep what you need by copying it.
328
329To advertise and accept STARTTLS (TLS 1.3, via
330[ianic/tls.zig](https://github.com/ianic/tls.zig)), pass a certificate key
331pair; the stream buffers must then be at least `zsmtp.tls.input_buffer_len` /
332`zsmtp.tls.output_buffer_len` bytes, since the handshake runs over them:
333
334```zig
335var auth: zsmtp.tls.config.CertKeyPair =
336 try .fromFilePath(gpa, io, .cwd(), "cert.pem", "key.pem");
337defer auth.deinit(gpa);
338
339var session: zsmtp.Server = .init(&stream_reader.interface, &stream_writer.interface, handler, .{
340 .hostname = "mx.example.com",
341 .tls = .{ .io = io, .auth = &auth },
342});
343try session.run(gpa);
344```
345
346On STARTTLS the session answers 220, performs the server handshake, swaps
347its transport to the encrypted connection, and resets state per RFC 3207 (the
348client must EHLO again). With `.mode = .implicit` the handshake instead runs
349before the greeting (SMTPS, port 465 style):
350
351```zig
352var session: zsmtp.Server = .init(&stream_reader.interface, &stream_writer.interface, handler, .{
353 .hostname = "mx.example.com",
354 .tls = .{ .io = io, .auth = &auth, .mode = .implicit },
355});
356```
357
358## Demo CLI
359
360```sh
361zig build
362
363# Debug server that prints received messages to stdout
364# (with a cert/key pair it advertises and accepts STARTTLS):
365./zig-out/bin/zsmtp serve 2525
366./zig-out/bin/zsmtp serve --tls-cert cert.pem --tls-key key.pem 2525
367./zig-out/bin/zsmtp serve --tls-cert cert.pem --tls-key key.pem --implicit-tls 2465
368
369# Send a message read from stdin:
370printf 'Subject: hi\r\n\r\nhello\r\n' | \
371 ./zig-out/bin/zsmtp send 127.0.0.1 2525 me@example.com you@example.net
372
373# Same, over implicit TLS or STARTTLS (--insecure skips cert verification):
374zsmtp send --tls smtp.example.com 465 me@example.com you@example.net
375zsmtp send --starttls smtp.example.com 587 me@example.com you@example.net
376
377# Send arbitrary binary content (RFC 3030), framed by BDAT rather than DATA:
378./zig-out/bin/zsmtp send --binarymime 127.0.0.1 2525 me@example.com you@example.net \
379 < some-binary-file
380
381# Speak LMTP (RFC 2033) instead of SMTP. The server reports one verdict per
382# recipient, and --fail-delivery makes one of them fail to show it:
383./zig-out/bin/zsmtp serve --lmtp --fail-delivery bad@example.net 2529
384printf 'Subject: hi\r\n\r\nhello\r\n' | \
385 ./zig-out/bin/zsmtp send --lmtp 127.0.0.1 2529 me@example.com \
386 good@example.net bad@example.net
387
388# Request a delivery status notification (RFC 3461):
389zsmtp send --ret hdrs --envid 'batch 7' --notify success,failure \
390 --orcpt team@example.net 127.0.0.1 2525 me@example.com you@example.net
391
392# Authenticate. Over a plaintext connection this refuses PLAIN and LOGIN
393# rather than put the password on the wire; --allow-cleartext-auth overrides
394# that for a connection protected by other means:
395zsmtp send --starttls --user me --password secret smtp.example.com 587 \
396 me@example.com you@example.net
397```
398
399## Status
400
401TLS is supported on both sides via
402[ianic/tls.zig](https://github.com/ianic/tls.zig): the client does implicit
403TLS and STARTTLS via `zsmtp.Tls`, and the server accepts both STARTTLS and
404implicit TLS (TLS 1.3 only). AUTH covers PLAIN, LOGIN, and CRAM-MD5 on the
405client and PLAIN and LOGIN on the server. Message bodies can be streamed on
406both sides, and the server validates MAIL and RCPT parameters (SIZE=, BODY=,
407and the DSN set RET=, ENVID=, NOTIFY=, ORCPT=). Both sides also speak LMTP,
408where a message ends with one verdict per recipient rather than one for the
409message, and both use PIPELINING, which collapses an envelope into a single
410round trip.
411
412## Known gaps
413
414Measured against the implementations people are likely to be coming from —
415Postfix, Exim and Haraka on the server side, Go's `net/smtp`, Python's
416`smtplib`, lettre and Nodemailer on the client side. Kept here so the list
417is one thing rather than a rediscovery each time.
418
419### Out of scope, not missing
420
421- **Message composition.** No MIME builder, headers, attachments, transfer
422 encodings, `Message-ID` or `Date` generation. zsmtp carries a message that
423 already exists; building one is RFC 5322's job and belongs in a library of
424 its own.
425- **DSN report generation**
426 ([RFC 3464](https://datatracker.ietf.org/doc/html/rfc3464)). The SMTP half
427 of DSN — RFC 3461's `RET`, `ENVID`, `NOTIFY` and `ORCPT` — is implemented
428 on both sides, but nothing here builds the `multipart/report` message that
429 carries a delivery status back to the sender. That is message composition
430 by another name, so it goes with the library above.
431- **Everything an MTA does around a session.** No queue, no retry schedule,
432 no MX resolution, no routing, no mailbox store. "Server" here means a
433 session handler: listening, accepting and concurrency are the caller's.
434
435### Protocol
436
437- **`AUTH=` on MAIL FROM** ([RFC 4954 §5](https://datatracker.ietf.org/doc/html/rfc4954#section-5)),
438 which a trusted relay uses to forward the identity that originally
439 authenticated. The client-side *mechanisms* are no longer a gap — PLAIN,
440 LOGIN, CRAM-MD5, EXTERNAL, XOAUTH2, OAUTHBEARER and SCRAM all come from
441 zig-sasl — but the **server** still understands only PLAIN and LOGIN, and
442 only against a plaintext password.
443- **Client certificates** — neither side can present or verify one.
444- **No enhanced status code accessor** — the server emits `x.y.z` on every
445 reply, but `Reply` exposes only `code` and the raw text.
446- `EXPN` is unrecognized rather than unimplemented, so it answers 500 where
447 [RFC 5321 §4.2.4](https://datatracker.ietf.org/doc/html/rfc5321#section-4.2.4)
448 wants 502.
449- Niche and absent: REQUIRETLS, MT-PRIORITY, DELIVERBY, FUTURERELEASE, ETRN.
450
451### Server
452
453- **No `Received:` header.**
454 [RFC 5321 §4.4](https://datatracker.ietf.org/doc/html/rfc5321#section-4.4)
455 requires a receiving server to stamp one.
456- **The handler never sees the connection** — no connect callback, no peer
457 address, no TLS state. Greylisting, DNSBLs, SPF and per-IP policy cannot
458 be built on top, and a `Received:` header cannot be written without it.
459- **No timeouts**, so a client that connects and says nothing holds the
460 session forever;
461 [RFC 5321 §4.5.3.2](https://datatracker.ietf.org/doc/html/rfc5321#section-4.5.3.2)
462 specifies per-command limits.
463- **No abuse limits** beyond `max_recipients`: unlimited failed AUTH
464 attempts, no error-count disconnect, no command budget.
465- **No `require_tls`** to go with `require_auth`.
466- **No PROXY protocol, XCLIENT or XFORWARD**, so the real peer address is
467 lost behind a load balancer.
468- No filter or milter hook, and so no DKIM, SPF, DMARC or ARC.
469- No logging or tracing hooks.
470- `max_message_size` is not enforced in `messageReader` mode.
471
472### Client
473
474- **`sendMail` is all-or-nothing on recipients** — a refused RCPT abandons
475 the transaction, where `smtplib.sendmail` delivers to the rest and reports
476 the refusals. `envelope` gives a caller the per-recipient codes to decide
477 for itself, but no higher-level call does that decision for it.
478- **No `SIZE=` on MAIL**, though the client parses the capability off EHLO:
479 `max_size` is read and never used, so nothing checks that a message fits
480 before transmitting it.
481- No MX resolution or connect helper, no 4xx retry or backoff, no connection
482 reuse helper.
483
484## Standards
485
486- [RFC 5321](https://datatracker.ietf.org/doc/html/rfc5321) — Simple Mail
487 Transfer Protocol: the command/reply protocol, multiline replies,
488 dot-stuffing, reply classes, and ESMTP parameter syntax (client and
489 server).
490- [RFC 1870](https://datatracker.ietf.org/doc/html/rfc1870) — SIZE:
491 advertised and enforced by the server (oversize declarations are rejected
492 with 552 before DATA); parsed from EHLO by the client.
493- [RFC 6152](https://datatracker.ietf.org/doc/html/rfc6152) — 8BITMIME:
494 advertised by the server and `BODY=` validated; parsed by the client.
495- [RFC 3030](https://datatracker.ietf.org/doc/html/rfc3030) — CHUNKING
496 (BDAT) and BINARYMIME: client and server, with length-based framing and no
497 dot-stuffing. `BODY=BINARYMIME` is advertised, accepted and delivered bit
498 for bit, and DATA is refused with 503 for a message that declared it,
499 since binary content cannot be framed by a line holding a single dot.
500- [RFC 3461](https://datatracker.ietf.org/doc/html/rfc3461) — DSN:
501 advertised by the server, which parses and validates `RET=`/`ENVID=` on
502 MAIL and `NOTIFY=`/`ORCPT=` on RCPT and hands them to the handler; the
503 client sends them through `mail`/`rcpt`. Includes the xtext codec of §4.
504 Generating the report message itself (RFC 3464) is out of scope.
505- [RFC 2033](https://datatracker.ietf.org/doc/html/rfc2033) — LMTP: client
506 and server, via `Client.mode` and `Server.Options.protocol`. `LHLO`
507 replaces `EHLO` and the end of a message draws one reply per accepted
508 recipient instead of one for the message, after DATA and after `BDAT
509 LAST` alike.
510- [RFC 2920](https://datatracker.ietf.org/doc/html/rfc2920) — PIPELINING:
511 the client sends a whole envelope as one group through `envelope`, and the
512 server holds back the replies it is allowed to (RSET, MAIL, RCPT) so they
513 leave together, sending everything pending the moment its input runs dry.
514- [RFC 3207](https://datatracker.ietf.org/doc/html/rfc3207) — STARTTLS:
515 client and server, including the mandatory post-handshake state reset.
516- [RFC 8314](https://datatracker.ietf.org/doc/html/rfc8314) — implicit TLS
517 (SMTPS): client (`Tls` before any SMTP traffic) and server
518 (`.mode = .implicit`).
519- [RFC 4954](https://datatracker.ietf.org/doc/html/rfc4954) — AUTH: client
520 and server, including initial responses, empty challenges and `*`
521 cancellation. The client drives any mechanism from
522 [zig-sasl](https://git.jcollie.dev/jeff/zig-sasl); the server still
523 implements PLAIN ([RFC 4616](https://datatracker.ietf.org/doc/html/rfc4616))
524 and the de-facto
525 [LOGIN](https://datatracker.ietf.org/doc/html/draft-murchison-sasl-login-00)
526 itself, because zig-sasl's server side does not yet reach past PLAIN.
527- [RFC 3463](https://datatracker.ietf.org/doc/html/rfc3463) /
528 [RFC 2034](https://datatracker.ietf.org/doc/html/rfc2034) — enhanced
529 status codes: carried in every server reply and advertised via
530 ENHANCEDSTATUSCODES; detected by the client.
531- [RFC 6531](https://datatracker.ietf.org/doc/html/rfc6531) — SMTPUTF8:
532 client (`mailFromUtf8`) and server (advertised; non-ASCII addresses
533 require the parameter and must be valid UTF-8, rejected with 553 5.6.7
534 per [RFC 6533](https://datatracker.ietf.org/doc/html/rfc6533) otherwise;
535 the flag reaches handlers via `Envelope.smtputf8`).
536
537TLS itself (TLS 1.3, [RFC 8446](https://datatracker.ietf.org/doc/html/rfc8446))
538is provided by [ianic/tls.zig](https://github.com/ianic/tls.zig).
539
540## References cited
541
542The specifications this implementation was written against, and the outside
543work it borrows from, in the RFC citation format so that a reference here
544matches one anywhere else. The **Standards** section above says what is
545implemented of each; this one says what each document *is*. Every entry is
546also filed in the project bibliography, so a citation can be taken from there
547rather than composed; the RFCs are keyed by their DOIs (`10.17487/RFC5321`
548and so on).
549
550- **[RFC1870]** Klensin, J., Freed, N., and K. Moore, "SMTP Service
551 Extension for Message Size Declaration", RFC 1870, November 1995,
552 <https://www.rfc-editor.org/info/rfc1870>.
553- **[RFC2033]** Myers, J., "Local Mail Transfer Protocol", RFC 2033,
554 October 1996, <https://www.rfc-editor.org/info/rfc2033>.
555- **[RFC2034]** Freed, N., "SMTP Service Extension for Returning Enhanced
556 Error Codes", RFC 2034, October 1996,
557 <https://www.rfc-editor.org/info/rfc2034>.
558- **[RFC2195]** Klensin, J., Catoe, R., and P. Krumviede, "IMAP/POP
559 AUTHorize Extension for Simple Challenge/Response", RFC 2195,
560 September 1997, <https://www.rfc-editor.org/info/rfc2195>.
561- **[RFC2920]** Freed, N., "SMTP Service Extension for Command Pipelining",
562 RFC 2920, September 2000, <https://www.rfc-editor.org/info/rfc2920>.
563- **[RFC3030]** Vaudreuil, G., "SMTP Service Extensions for Transmission of
564 Large and Binary MIME Messages", RFC 3030, December 2000,
565 <https://www.rfc-editor.org/info/rfc3030>.
566- **[RFC3207]** Hoffman, P., "SMTP Service Extension for Secure SMTP over
567 Transport Layer Security", RFC 3207, February 2002,
568 <https://www.rfc-editor.org/info/rfc3207>.
569- **[RFC3461]** Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
570 Extension for Delivery Status Notifications (DSNs)", RFC 3461,
571 January 2003, <https://www.rfc-editor.org/info/rfc3461>.
572- **[RFC3463]** Vaudreuil, G., "Enhanced Mail System Status Codes",
573 RFC 3463, January 2003, <https://www.rfc-editor.org/info/rfc3463>.
574- **[RFC3464]** Moore, K. and G. Vaudreuil, "An Extensible Message Format
575 for Delivery Status Notifications", RFC 3464, January 2003,
576 <https://www.rfc-editor.org/info/rfc3464>. *(Cited as out of scope: the
577 report message itself.)*
578- **[RFC4616]** Zeilenga, K., "The PLAIN Simple Authentication and Security
579 Layer (SASL) Mechanism", RFC 4616, August 2006,
580 <https://www.rfc-editor.org/info/rfc4616>.
581- **[RFC4954]** Siemborski, R. and A. Melnikov, "SMTP Service Extension for
582 Authentication", RFC 4954, July 2007,
583 <https://www.rfc-editor.org/info/rfc4954>.
584- **[RFC5321]** Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
585 October 2008, <https://www.rfc-editor.org/info/rfc5321>.
586- **[RFC5322]** Resnick, P., Ed., "Internet Message Format", RFC 5322,
587 October 2008, <https://www.rfc-editor.org/info/rfc5322>. *(Cited as out
588 of scope: the format of the message this library carries.)*
589- **[RFC6152]** Klensin, J., Freed, N., Rose, M., and D. Crocker, "SMTP
590 Service Extension for 8-bit MIME Transport", RFC 6152, March 2011,
591 <https://www.rfc-editor.org/info/rfc6152>.
592- **[RFC6531]** Yao, J. and W. Mao, "SMTP Extension for Internationalized
593 Email", RFC 6531, February 2012,
594 <https://www.rfc-editor.org/info/rfc6531>.
595- **[RFC6533]** Hansen, T., Ed., Newman, C., and A. Melnikov,
596 "Internationalized Delivery Status and Disposition Notifications",
597 RFC 6533, February 2012, <https://www.rfc-editor.org/info/rfc6533>.
598- **[RFC7628]** Mills, W., Showalter, T., and H. Tschofenig, "A Set of
599 Simple Authentication and Security Layer (SASL) Mechanisms for OAuth",
600 RFC 7628, August 2015, <https://www.rfc-editor.org/info/rfc7628>.
601 *(Cited as a gap.)*
602- **[RFC7677]** Hansen, T., "SCRAM-SHA-256 and SCRAM-SHA-256-PLUS Simple
603 Authentication and Security Layer (SASL) Mechanisms", RFC 7677,
604 November 2015, <https://www.rfc-editor.org/info/rfc7677>. *(Cited as a
605 gap.)*
606- **[RFC8314]** Moore, K. and C. Newman, "Cleartext Considered Obsolete:
607 Use of Transport Layer Security (TLS) for Email Submission and Access",
608 RFC 8314, January 2018, <https://www.rfc-editor.org/info/rfc8314>.
609- **[RFC8446]** Rescorla, E., "The Transport Layer Security (TLS) Protocol
610 Version 1.3", RFC 8446, August 2018,
611 <https://www.rfc-editor.org/info/rfc8446>.
612- **[SASL-LOGIN]** Murchison, K. and M. Crispin, "The LOGIN SASL
613 Mechanism", Work in Progress, Internet-Draft,
614 draft-murchison-sasl-login-00, August 2003,
615 <https://datatracker.ietf.org/doc/html/draft-murchison-sasl-login-00>.
616 The draft expired and LOGIN was never standardized; it is implemented
617 here because servers still ask for it.
618- **[TLS.ZIG]** Ianic, "tls.zig — TLS 1.2/1.3 implementation in Zig",
619 <https://github.com/ianic/tls.zig>. Provides the TLS on both sides; see
620 the **TLS** section for why the standard library's client is not used.
621- **[ISEMAIL]** Sayers, D., "is_email — an email address validator and its
622 test suite", BSD-3-Clause, <https://github.com/dominicsayers/isemail>.
623 The address corpus the path parser is checked against; see **Tests**.
624- **[EXIM]** The Exim Maintainers, "Exim Internet Mailer",
625 GPL-2.0-or-later, <https://www.exim.org/>. The protocol torture script
626 and the gauntlet unit test's dialogue are adapted from its test suite.
627
628## Tests
629
630```sh
631zig build test
632zig build test --fuzz # run the fuzz tests under the fuzzer (endless)
633```
634
635The fuzz tests cover parser crash-safety (`Command.parse`, `Reply.read`),
636whole-session robustness against arbitrary bytes on both the client and
637server side, and two differential properties: the streaming `DataWriter`
638must produce byte-identical output to the slice-based `writeStuffed` under
639fuzzer-chosen chunk boundaries, and the collecting and streaming server
640DATA paths must yield identical message content.
641
642### Protocol torture testing with exim's test client
643
644Exim's scriptable SMTP test client (`test/src/client.c` in the exim
645source) sends raw protocol lines and asserts reply prefixes. The exim
646source is declared as a *lazy* Zig dependency, fetched only on demand:
647
648```sh
649zig build -Dexim-client # fetches exim, installs zig-out/bin/exim-client
650./zig-out/bin/zsmtp serve 2525 &
651./zig-out/bin/exim-client 127.0.0.1 2525 < test/protocol-torture.script
652```
653
654### Address corpus testing with the is_email suite
655
656Dominic Sayers' [is_email](https://github.com/dominicsayers/isemail) test
657suite (BSD-3-Clause) is declared as a *lazy* Zig dependency; nothing from
658it is copied into this repository. On demand, the corpus test embeds its
659XML test files, extracts the 125 addresses valid at the RFC 5321 layer,
660and checks that each passes through the path parser byte-for-byte:
661
662```sh
663zig build test -Disemail-corpus # fetches the suite and runs the corpus test
664```
665
666Without the option the corpus test is skipped.
667
668`test/protocol-torture.script` is a 28-reply dialogue distilled from
669exim's own test suite (syntax errors, sequencing violations, parameter
670validation, dot-stuffing); the same dialogue is asserted byte-for-byte
671as a unit test in `Server.zig`.
672
673The library is MIT-licensed; the small amount of test-only material adapted
674from exim's test suite (the torture script and the gauntlet unit test's
675dialogue) is GPL-2.0-or-later, marked with SPDX snippet tags and REUSE.toml
676annotations.
677
678Note: Zig 0.16.0's fuzz *driver* is broken out of the box (its bundled
679test runner fails to compile in fuzz mode, and the coverage server panics
680on a test binary with no fuzz tests); both are fixed on Zig master. Until
681then, fuzzing needs a patched copy of the standard library via
682`zig build --zig-lib-dir <patched-lib> test --fuzz`. The fuzz tests
683themselves also run once per invocation as part of the normal
684`zig build test` suite.
685
686Interoperability against third-party implementations is covered by a NixOS
687VM test (`nix/interop-test.nix`): the zsmtp client delivers mail to Postfix
688and Exim over plaintext, STARTTLS, and implicit TLS against each, and swaks
689delivers to the zsmtp server over plaintext and STARTTLS.
690
691```sh
692nix build .#zsmtp # build the package
693nix build .#checks.x86_64-linux.interop # run the VM interop test
694```