If you attempt to enable IPv6 in your Dovecot configuration through WHM, you may see that it fails to start with this error:

Log Messages:
Jan 14 18:12:04 host dovecot: Fatal: listen: Invalid port in address :
Jan 14 18:11:13 host dovecot: Fatal: listen: Invalid port in address :
Jan 14 18:11:12 host dovecot: auth(default): Killed with signal 15 (by pid=19689 uid=0 code=kill)
Jan 14 18:11:12 host dovecot: Killed with signal 15 (by pid=19689 uid=0 code=kill)

If you see that error, it typically means that your dovecot version is out of date. In WHM 11.46, you should be running Dovecot version 2.x+. If you are not, it typically means that you have your dovecot version set to not be managed by cPanel:

[root@host ~]cPs# rpm -q dovecot
dovecot-1.2.17-0cp
[root@host ~]cPs# grep dovecot /var/cpanel/rpm.versions.d/local.versions
dovecot: unmanaged
[root@host ~]cPs#

The quickest way to resolve this is by allowing cPanel to manage your Dovecot rpm, by removing that setting, and having cPanel update the rpm:

/scripts/update_local_rpm_versions --del target_settings.dovecot
/scripts/check_cpanel_rpms --fix

Note: If you perviously enabled IPv6 for Dovecot, you will need to disable it before cPanel will be able to update the Dovecot version/RPM.