srmilter

A Rust library for building mail filter (milter) daemons that integrate with Postfix.

Overview

srmilter implements the milter protocol to receive emails from Postfix, parse them, and return classification decisions (accept, reject, or quarantine). It provides a simple API for writing custom email classifiers.

Features

• Milter protocol implementation for Postfix integration
• Email parsing via mail-parser crate
• Multiple concurrency modes: single-threaded, forked processes, or threaded
• Spamhaus ZEN DNSBL lookup utilities
• systemd socket activation support (optional)
• Built-in CLI with test and dump commands

Usage

1. Create a new binary crate:

   cargo init myfilter
   cd myfilter
   

2. Add srmilter as a dependency:

   cargo add srmilter
   

3. Edit src/main.rs to create your milter binary (see example below).

Example

use srmilter::{ClassifyResult, Config, EmailClassifier, MailInfo, read_array, array_contains};

struct MyContext {
    blocklist: Vec<String>,
}

fn main() -> impl std::process::Termination {
    let ctx = MyContext {
        blocklist: read_array("/etc/srmilter/blocklist.txt").unwrap_or_default(),
    };
    let classifier = EmailClassifier::builder(ctx).classify_fn(classify).build();
    let config = Config::builder()
        .email_classifier(classifier)
        .enable_fork_mode()
        .build();
    srmilter::cli::cli(&config)
}

fn classify(ctx: &MyContext, mail_info: &MailInfo) -> ClassifyResult {
    if array_contains(&ctx.blocklist, mail_info.get_from_address()) {
        return mail_info.reject("sender on blocklist");
    }
    mail_info.accept("default")
}

CLI Commands

The built-in CLI provides three subcommands:

# Run the milter daemon (default: 0.0.0.0:7044)
myfilter daemon [address] [--fork N] [--threads N] [--truncate N]

# Test classifier against an .eml file
myfilter test <file.eml> [sender] [recipients...]

# Dump parsed email headers and body
myfilter dump <file.eml> [-H] [-b] [--html]

Concurrency Options

• Default: Single-threaded, sequential processing
• --fork N: Fork up to N child processes (requires enable_fork_mode())
• --threads N: Use up to N threads

Postfix Configuration

Add to your Postfix main.cf:

smtpd_milters = inet:127.0.0.1:7044

License

Copyright © 2025 Donald Buczek buczek@molgen.mpg.de

Licensed under the European Union Public Licence (EUPL), Version 1.2. See the LICENSE file for details.