Sieve configuration with postfix and dovecot on Ubuntu 20.04

With a existing Postfix and Dovecot configuration on a Ubuntu 20.04 server, I required the following changes to get sieve server based mail filtering to work:

Install dovecot-sieve

To install the dovecot sieve plugin run

apt install dovecot-sieve

Change the mailbox transport to lmtp

Edit /etc/postfix/main.cf to set the line

 mailbox_transport = lmtp:unix:private/dovecot-lmtp

If dovecot is configured to use local usernames (without @domain on the end), you also need to edit /etc/dovecot/conf.d/10-auth.conf to add this line:

 auth_username_format = %Ln

This means that for the username dovecot will use the lowercase part to the left of the "@".

Restart daemons

Restart postfix and dovecot so they use the new config, and then check their status:

systemctl restart postfix dovecot
systemctl status postfix dovecot

Enable the sieve plugin in dovecot

Add this section to /etc/dovecot/conf.d/20-lmtp.conf:

protocol lmtp {
    mail_plugins = $mail_plugins sieve
}

 Configure a user's sieve filters

The default location for a user's sieve filters is configured in /etc/dovecot/conf.d/90-sieve.conf as follows:

plugin {
    sieve = file:~/sieve;active=~/.dovecot.sieve
}

So, as yourself (not root) run:

cd
mkdir sieve
touch default.sieve
ln -s sieve/default.sieve .dovecot.sieve

Then edit ~/sieve/default.sieve with your sieve configuration, for example:

require ["fileinto"};
if header :contains "Subject" "test" {
    fileinto "Test";
}

For full details about sieve filters see see RFC5228 Sieve: An Email Filtering Language.

Test

Send yourself an email with "test" (lowercase) in the subject. It should end up in the Test mail folder.

If the test mail isn't filtered check ~/.dovecot.sieve.log and /var/log/mail.log for problems.