Rockmail is a translation of Procmail into Rust. Procmail is a Mail Delivery Agent (MDA) for filtering, sorting, and delivering mail on Unix systems.
The goal is a near-100% backward-compatible, drop-in, "rock solid"
replacement for Procmail. Most existing .procmailrc files should work
without modification. A small number of features are deliberately not
implemented and a small number of differences exist. They are documented
in COMPATIBILITY.md.
Some extensions beyond Procmail are also available; see
ENHANCEMENTS.md.
This software has been translated from C into Rust using AI, but it is not "vibe coded." The AI worked from the original C source code, and a human operator was present at all times and provided guidance as needed. All of the code has been reviewed by a human, though that does not mean it is correct; several defects were initially unnoticed by the human.
Additionally, large parts of the code have been tested only by machine, not by humans in actual usage. Sorry, but I do not use every feature that Procmail has, many of which I did not even know about. Some features are not even documented. I was very inclusive when porting and only omitted features with security problems or that I suspected had not been used by anyone anywhere for multiple decades.
I cannot test all of that myself, but see notes on testing below to read how I have attempted to bridge the gap.
I am not going to tell you what is tested by my personal setup and what is not; it is better that you pay very close attention if you install this software. Better would be to audit it yourself!
Build it, then feed mail into it, just as you would Procmail.
If you need further instructions, you probably should not use this software.
THIS SOFTWARE HAS SEEN LIMITED PRODUCTION USE, AND IT SHOULD BE DEPLOYED CAREFULLY. DO NOT SWAP IT IN AS YOUR PROCMAIL REPLACEMENT WITHOUT TESTING!
DO NOT INSTALL THIS SOFTWARE SETUID ROOT! Root privileges are not needed! Root privileges will not be dropped! Rockmail expects that the MTA will setuid to the destination user before starting delivery.
Formail has been removed due to security issues. Procmail idioms tend to pass untrusted input through the shell when piping to formail, creating command injection risks. Its functionality is replaced by native rcfile syntax (see below).
Notable extensions:
- Header manipulation (
@I,@i,@a,@A) — modify headers on the in-flight message without forking a subprocess, replacingformail -I,-i,-a,-A. - Duplicate detection (
@D) — check Message-ID against a cache file, replacingformail -D. - Regex substitution (
=~) — applys/pattern/replacement/flagsto a variable without invoking a shell. - RFC 2047 decoding — encoded headers are decoded during condition matching, so patterns can match the decoded text directly. Manipulated headers are automatically RFC 2047 encoded if they contain non-ASCII.
- Pretty-printed errors — parse errors are rendered with source context
and color via the
miettecrate if running in a terminal.
See COMPATIBILITY.md for the full list of intentional behavioral
differences.
Test coverage exceeds 97% as of 2026-02-21 as measured by cargo llvm-cov. Most of the coverage gaps are in error situations that are
difficult to test in automation.
On 2026-02-22, the following interesting statistics were observed:
- 12K lines of C in the original Procmail (as of version 3.24)
- 22K lines of Rust code
- of which 14K lines are tests (compared to zero in Procmail)
- for 8K lines of net software
(But remember what the original code contains, to wit: custom memory management, string manipulation, a regular expression engine, etc. Although once necessary, all of that code can now, 35 years later, be left behind thanks to the Rust standard library and crate ecosystem.)
On 2026-02-22, the 1043 tests that were present comprised:
- 857 unit tests
- 56 integration tests
- 126 gold tests
- 4 regression tests
"Gold" means an integration test that compares Rockmail output to Procmail output (the gold standard) and ensures they are byte identical.
During initial development, gold testing revealed one minor bug in Procmail!
If running the tests yourself, it would be well to:
- Use
cargo nextest runinstead ofcargo test - Compile a custom Procmail with the ssleep set to zero in
mailfold.c
These two steps speed up the tests by a factor of five.
Most of this repository is a translation of Procmail's C code into Rust. That translated code is a work derived from Procmail, so the license for that code and the default license for code in this repository is GPLv2, the same as for Procmail.
Some smaller parts of the code are NOT derivatives and are therefore subject to a license chosen by me, which is the included 3-clause BSD license.
Procmail contains a shell script with the same name as this binary, but due to bugs in it, I wrote my own version in Python between 2000 and 2002 without making any reference to the original shell script.
This binary is a translation into Rust of that Python code, NOT of the original shell script. The original shell script was not used in any way to produce this Rust code.
If the nfs feature is NOT used, this program may be used under the terms
of the included BSD license. If nfs is enabled, then the compiled program
is infected by GPL restrictions and may be used only under GPL restrictions.
This module is an alternative locking implementation designed for local filesystems only, instead of Procmail's NFS-oriented algorithm.
Total token consumption from the beginning to a reasonable level of
completeness at commit 75cdcaa0 was approximately 1.28 billion tokens,
which would have cost $740 at API prices.
Sidney Cammeresi
[email protected]