FreeBSD email server - Part 5: Filtering mail

FreeBSD email server - Part 5: Filtering mail

Being able to send mail and not be flagged as spam is pretty awesome on itself. But you also get hit by a lot of spam. The more you give out your email address and domain name, the more spam you will receive over time. I welcome you to another part of the FreeBSD email server series. In this part, we will set up email filtering at the server side.

We will accomplish this with a couple packages, SpamAssassin and Pigeonhole. The former deals with scanning the emails to deduce whether it is spam or not. The latter filters messages. We will use this filtering to drop emails marked as spam by SpamAssassin into the Junk folder, instead of the inbox.

Installing the packages

Both packages are available through FreeBSD's pkg utility. Install them as such.

pkg install dovecot-pigeonhole spamassassin

SpamAssassin

Enabling the service

Like most services, you have to enable them as well. Pigeonhole is an extension to Dovecot, and Dovecot will handle this one. SpamAssassin requires you to configure the service as well. You can enable it and set sane configuration to it with the following two commands.

echo 'spamd_enable="YES"' >> /etc/rc.conf.local
echo 'spamd_flags="-u spamd -H /srv/mail"' >> /etc/rc.conf.local

Acquiring default spam rules

SpamAssassin has to "learn" what counts as spam and what counts as ham. To fetch these rules, you should execute the updates for SpamAssassin with the following command.

sa-update

You most likely want to run this once every while, so it is advised to setup a cronjob for this purpose.

Postfix

In order to have mails checked by SpamAssassin, Postfix must be instructed to pass all email through to SpamAssassin, which will hand them back with a X-Spam-Flag header attached to them. This header can be used by other applications to treat it as spam.

master.cf

There's not much to include to the already existing Postfix configuration to enable SpamAssassin to do its job. Just open /usr/local/etc/postfix/master.cf and append the block given below.

spamassassin  unix  -       n       n       -       -       pipe
  user=spamd argv=/usr/local/bin/spamc
  -f -e /usr/sbin/sendmail -oi -f ${sender} ${recipient}

Pigeonhole

Pigeonhole is an implementation of Sieve for Dovecot. It deals with filtering messages on the server side using a set of rules, defined in a file usually named sieve. This file is generally saved at /srv/mail/domain.tld/user/sieve. A default file to filter spam out is the following example.

require [
    "fileinto",
    "mailbox"
];

if header :contains "X-Spam-Flag" "YES" {
    fileinto :create "Junk";
    stop;
}

This looks for the X-Spam-Flag header, which is added by SpamAssassin. If it is set to YES, this indicates SpamAssassin thinks the message is spam. As such, sieve is instructed to filter this message into the folder Junk, and to create this folder if it does not exist yet. The stop; makes sieve stop trying to process this message further based on later rules.

Dovecot

Dovecot needs some additional configuration to work with Pigeonhole. Modify the following files and add the contents described.

conf.d/20-lmtp.conf

This will enable Pigeonhole in Dovecot.

protocol lmtp {
  mail_plugins = $mail_plugins sieve
}

conf.d/90-plugin.conf

This configures Pigeonhole to look for a file named sieve in the mailbox homedir, and execute that when delivering mail.

plugin {
  sieve = /srv/mail/%d/%n/sieve
}

Conclusion

Spam is a pain, especially if you get a lot of it. The configuration added in this part of the FreeBSD email server series should get rid of most of it. This also concludes the series. If you have any questions or suggestions, please contact me via any of the methods detailed on my home page.

Thanks for reading along, and enjoy your very own email server!