Symptoms

On a just deployed QMail host the Dovecot service cannot be started:

# service dovecot restart
Stopping Dovecot Imap:                                     [FAILED]
Starting Dovecot Imap: If you have trouble with authentication failures,
enable auth_debug setting. See http://wiki.dovecot.org/WhyDoesItNotWork
This message goes away after the first successful login.
Error: socket() failed: Address family not supported by protocol
Fatal: listen(::, 143) failed: Address family not supported by protocol
                                                           [FAILED]

Cause

IPv6 is disabled on the host.

Resolution

In the /etc/dovecot.conf file set the listen parameter to *. For example: listen = *. Then, restart the service.