An SMTP client and server library for Zig implementing RFC 5321.
Add exim as a lazy dependency to build its SMTP test client
The exim source (pinned commit) is declared with .lazy = true and only
fetched when requested: `zig build -Dexim-client` compiles exim's
scriptable SMTP test client (test/src/client.c) to
zig-out/bin/exim-client. The option guard keeps plain builds, tests,
and the network-less Nix sandbox build from ever fetching it.
test/protocol-torture.script (with a REUSE.toml annotation) carries
the 28-reply dialogue distilled from exim's test suite; running
exim-client with it against zsmtp serve passes all expectations, and
the same dialogue is asserted byte-for-byte by the protocol gauntlet
unit test.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012HBHFhoTYa8TU9GLwobfbx
author
Jeffrey C. Ollie
co-author
Claude Fable 5
date
(Aug 30, 2026, 1:49 AM -0500)
commit
b370fc0e
b370fc0e8411a3e3d6bbf961c4a6b8d5c7d078d1
parent
dcc9aacd
dcc9aacdb3e292699661f9de2c6dc120ad2d1b4f
+112
+17
README.md
+17
README.md
···
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fuzzer-chosen chunk boundaries, and the collecting and streaming server
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DATA paths must yield identical message content.
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### Protocol torture testing with exim's test client
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Exim's scriptable SMTP test client (`test/src/client.c` in the exim
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source) sends raw protocol lines and asserts reply prefixes. The exim
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source is declared as a *lazy* Zig dependency, fetched only on demand:
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```sh
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zig build -Dexim-client # fetches exim, installs zig-out/bin/exim-client
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./zig-out/bin/zsmtp serve 2525 &
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./zig-out/bin/exim-client 127.0.0.1 2525 < test/protocol-torture.script
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```
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`test/protocol-torture.script` is a 28-reply dialogue distilled from
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exim's own test suite (syntax errors, sequencing violations, parameter
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validation, dot-stuffing); the same dialogue is asserted byte-for-byte
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as a unit test in `Server.zig`.
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Note: Zig 0.16.0's fuzz *driver* is broken out of the box (its bundled
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test runner fails to compile in fuzz mode, and the coverage server panics
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on a test binary with no fuzz tests); both are fixed on Zig master. Until
+5
REUSE.toml
+5
REUSE.toml
···
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path = "flake.lock"
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SPDX-FileCopyrightText = "© 2026 Jeffrey C. Ollie <jeff@ocjtech.us>"
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SPDX-License-Identifier = "MIT"
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[[annotations]]
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path = "test/protocol-torture.script"
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SPDX-FileCopyrightText = "© 2026 Jeffrey C. Ollie <jeff@ocjtech.us>"
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SPDX-License-Identifier = "MIT"
+23
build.zig
+23
build.zig
···
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run_cmd.addArgs(args);
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}
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// Exim's scriptable SMTP test client, handy for driving the zsmtp
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// server through raw protocol dialogues with reply expectations (see
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// test/protocol-torture.script). Guarded by an option so the lazy exim
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// dependency is only fetched on demand:
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// zig build -Dexim-client && ./zig-out/bin/exim-client <host> <port>
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if (b.option(bool, "exim-client", "Build exim's scriptable SMTP test client (fetches the exim source)") orelse false) {
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if (b.lazyDependency("exim", .{})) |exim_dep| {
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const exim_client = b.addExecutable(.{
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.name = "exim-client",
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.root_module = b.createModule(.{
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.target = target,
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.optimize = optimize,
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.link_libc = true,
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}),
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});
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exim_client.root_module.addCSourceFile(.{
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.file = exim_dep.path("test/src/client.c"),
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.flags = &.{"-w"},
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});
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b.installArtifact(exim_client);
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}
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}
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// Creates an executable that will run `test` blocks from the provided module.
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// Here `mod` needs to define a target, which is why earlier we made sure to
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// set the releative field.
+7
build.zig.zon
+7
build.zig.zon
···
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.url = "https://github.com/ianic/tls.zig/archive/e04ae448ce7ee70c136d4d48b059314543203809.tar.gz",
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.hash = "tls-0.1.0-ER2e0jGpBgCkVC-Yp12NgSdHNUtZr52MleJ8roHlUa54",
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},
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// Exim's source, used only for its scriptable SMTP test client
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// (test/src/client.c). Lazy: fetched only by `zig build -Dexim-client`.
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.exim = .{
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.url = "https://github.com/Exim/exim/archive/13835a3c1e057efad7da269c0f93bf2eac850205.tar.gz",
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.hash = "N-V-__8AAFtMiwHRrbE2kejFyc1FTFciBTZJlJOPGt-eIVGJ",
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.lazy = true,
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},
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},
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.paths = .{
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"build.zig",
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test/protocol-torture.script
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test/protocol-torture.script
···
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??? 220
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NOOP
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??? 250
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rhubarb
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??? 500
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mail from:<x@y>
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??? 503
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rcpt to:<a@b>
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??? 503
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ehlo test.client
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??? 250-
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??? 250-PIPELINING
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??? 250-8BITMIME
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??? 250-ENHANCEDSTATUSCODES
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??? 250 SIZE
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mail
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??? 501
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mail from:
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??? 501
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mail from:<>
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??? 250
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mail from:<x@y>
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??? 503
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rcpt to:
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??? 501
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data
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??? 503
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rset
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??? 250
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etrn abc
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??? 500
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vrfy userx
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??? 252
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help
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??? 214
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mail from:<ok@test1> SIZE=100 BODY=8BITMIME
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??? 250
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rcpt to:<userx@test.ex>
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??? 250
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rcpt to:<@relay.example:route@test.ex>
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??? 250
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data
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??? 354
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..that line started with a dot
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.. and one starting with two dots
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Message body
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.
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??? 250
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mail from:<a@b> SIZE=99999999
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??? 552
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mail from:<a@b> BODY=BINARYMIME
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??? 555
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mail from:<a@b> FOO=bar
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??? 555
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mail from:<a@b> SIZE=nan
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??? 501
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starttls
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??? 502
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quit
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??? 221