Dovecot is a powerful mailbox manager supporting imap, imaps, pop and a multitude of plugins. It is frequently used in combination with postfix - where postfix acts as an MTA, dovecot manages the incoming e-mail, as well as serving it over different protocols. This guide explains how to use sive, a dovecot plugin, that can be used for e-mail triage.

Under Debian, postfix uses procmail to deliver e-mail but we can switch to dovecot's deliver agent and get rid of procmail altogether.

Dovecot needs the lda protocol enabled. This can be achieved by editing /etc/dovecot/dovecot.conf and enabling the following options:

protocol lda {
  # Address to use when sending rejection mails (e.g. postmaster@example.com).
  postmaster_address = postmaster@example.tld

  # Hostname to use in various parts of sent mails, eg. in Message-Id.
  # Default is the system's real hostname.
  hostname = example.tld
  
  # Support for dynamically loadable plugins. mail_plugins is a space separated
  # list of plugins to load.
  mail_plugins = sieve
  mail_plugin_dir = /usr/lib/dovecot/modules/lda 

  # Subject: header to use for rejection mails. You can use the same variables
  # as for rejection_reason below.
  rejection_subject = Rejected: %s

  # Human readable error message for rejection mails. You can use variables:
  #  %n = CRLF, %r = reason, %s = original subject, %t = recipient
  rejection_reason = Your message to <%t> was automatically rejected:%n%r

  # UNIX socket path to master authentication server to find users.
  auth_socket_path = /var/run/dovecot/auth-master
}

where example.tld is the hostname for which dovecot is managing e-mails.

Next, we go all the way down to the plugin section of dovecot.conf and enable sieve:

  # Sieve plugin (http://wiki.dovecot.org/LDA/Sieve) and ManageSieve service
  #
  # Location of the active script. When ManageSieve is used this is actually
  # a symlink pointing to the active script in the sieve storage directory.
  sieve=~/.dovecot.sieve

This configuration setting will allow each user for which e-mail is served to create their own .dovecot.sieve file in their home directory in order to redirect e-mail to various folders.

Now, we can get rid of procmail by editing /etc/postfix/main.cf and replacing the line:

mailbox_command = procmail -a "$EXTENSION"

with:

mailbox_command = /usr/lib/dovecot/deliver

which will now make dovecot handle the actual delivery of e-mails to their corresponding destinations.

After this step is done, one can optionally remove procmail from the system and thereby reduce dependencies:

dpkg --purge procmail

Now that dovecot is handling the delivery, and that sieve is going to read the .dovecot.sieve in each user's home directory, we can experiment with some sample rules:

require "fileinto";
if allof (address :is "from" "service@mail.secondlife.com",
          header :contains "Subject" "Customer") {
        fileinto "marketplace";
}
if anyof (address :contains "from" "im.agni.lindenlab.com",
                address :is "from" "service@secondlife.com") {
        fileinto "secondlife";
}
if header :contains "Subject" "Cydia/APT(A)" {
        fileinto "cydia";
}

the example above will do the following:

• if the from field is exactly service@mail.secondlife.com, and (alloff) the subject field contains the word Customer then that e-mail will be stored in a folder called marketplace.
• if the from field contains im.agni.lindenlab.com, or (anyof) the address is exactly service@secondlife.com then that e-mail will be stored in a folder called secondlife.

and finally:

• if the subject field contains Cydia/APT(A) then that e-mail will be stored in a folder called cydia.

Note that these folders are created automatically if they do not exist.