Documentation

Getting Started with MIMEDefang

MIMEDefang is a mail filtering framework that hooks into Sendmail or Postfix via the Milter API. You write your filtering logic in Perl; MIMEDefang handles the integration with the MTA.

Prerequisites

  • MTA – Sendmail 8.12+ with milter support, or Postfix 2.3+ with milter support enabled.
  • Perl – Perl 5.10 or later with the MIME::Tools distribution installed.
  • Root access – required for installation and service management.

Installation

From a package manager (recommended)

On Fedora / RHEL / CentOS:

dnf install mimedefang

On Debian / Ubuntu:

apt-get install mimedefang

From source

./configure
make
make install

The default installation places binaries in /usr/bin/ and looks for the filter script at /etc/mail/mimedefang-filter.

Your First Filter

MIMEDefang reads its filtering logic from a Perl file called mimedefang-filter. A minimal filter that accepts every message looks like this:

sub filter_end {
    return action_accept();
}

1;

Save this as /etc/mail/mimedefang-filter. You can test it before starting the daemon:

mimedefang.pl -f /etc/mail/mimedefang-filter /path/to/test/dir

This runs your filter offline against a message directory and reports any errors.

Starting the Service

MIMEDefang consists of two processes. The multiplexor manages a pool of Perl workers; the main daemon communicates with the MTA.

With systemd

systemctl enable --now mimedefang

Manually

Start the multiplexor first, then the daemon:

mimedefang-multiplexor -p /var/run/mimedefang/mimedefang-multiplexor.sock \
    -f /etc/mail/mimedefang-filter

mimedefang -p inet:8998@localhost \
    -m /var/run/mimedefang/mimedefang-multiplexor.sock \
    -U defang

Consult the mimedefang(8) and mimedefang-multiplexor(8) man pages for the full list of options.

Telling Your MTA to Use the Filter

Sendmail

Add this line to your sendmail.mc and rebuild:

INPUT_MAIL_FILTER(`mimedefang', `S=inet:8998@localhost, F=T, T=S:360s;R:360s;E:15m')

Postfix

Postfix communicates with MIMEDefang through the milter (mail filter) interface. You need Postfix 2.3 or later.

1. Choose a socket type

MIMEDefang can listen on either a TCP socket or a Unix socket.

TCP (simpler, works across hosts):

mimedefang -p inet:8998@127.0.0.1 \
    -m /var/run/mimedefang/mimedefang-multiplexor.sock \
    -U defang

Unix socket (lower overhead, same host only):

mimedefang -p local:/var/run/mimedefang/mimedefang.sock \
    -m /var/run/mimedefang/mimedefang-multiplexor.sock \
    -U defang

2. Register the milter in main.cf

For a TCP socket:

smtpd_milters = inet:127.0.0.1:8998
non_smtpd_milters = inet:127.0.0.1:8998
milter_default_action = accept

For a Unix socket:

smtpd_milters = unix:/var/run/mimedefang/mimedefang.sock
non_smtpd_milters = unix:/var/run/mimedefang/mimedefang.sock
milter_default_action = accept
  • smtpd_milters applies the filter to mail received over SMTP.
  • non_smtpd_milters applies it to locally submitted mail (e.g. from sendmail or pickup). Omit this line if you only want to filter inbound SMTP.
  • milter_default_action = accept tells Postfix to accept mail if MIMEDefang is unavailable, avoiding a mail outage during restarts.

3. Reload Postfix

postfix reload

Postfix will now pass every incoming message through MIMEDefang before delivery.

Next Steps