Description of problem:

Dovecot fails to start after a reboot as it cannot bind to the specified IP addresses. The relevant part in /var/log/messages:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Feb 19 03:28:36 mail systemd: Started Dovecot IMAP/POP3 email server.
Feb 19 03:28:36 mail dovecot: Error: bind(IP_ADDR_HERE, 995) failed: Cannot assign requested address
Feb 19 03:28:36 mail dovecot: Error: service(pop3-login): listen(mail.example.net, 995) failed: Cannot assign requested address
Feb 19 03:28:36 mail dovecot: Error: bind(IP_ADDR_HERE, 993) failed: Cannot assign requested address
Feb 19 03:28:36 mail dovecot: Error: service(imap-login): listen(mail.example.net, 993) failed: Cannot assign requested address
Feb 19 03:28:36 mail dovecot: Fatal: Failed to start listeners
Feb 19 03:28:36 mail systemd: dovecot.service: main process exited, code=exited, status=89/n/a
Feb 19 03:28:36 mail systemd: Unit dovecot.service entered failed state.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Version-Release number of selected component (if applicable):
~]# rpm -q dovecot
dovecot-2.2.10-5.el7.x86_64

How reproducible:

Reproducible on every reboot.

Additional info:

The resolution involves fixing the ordering of dependencies and setting the dovecot service to start after the network has entered the online state.

Helpful instructions how to overcome the issue for anyone who reads this bug report:

1. Create a file at /etc/systemd/system/dovecot.service with the following contents:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.include /usr/lib/systemd/system/dovecot.service

[Unit]
After=network-online.target
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

2. Issue the command:

~]# systemctl daemon-reload

or reboot.

Note: All credit for the workaround goes to the wise folks at #centos.