dovecot can't start when IPv6 is disabled.

Issue

• dovecot can't start if IPv6 is disabled

  # vi /etc/modprobe.d/blacklist
  -------------------------------------------
  install ipv6 /bin/true
  -------------------------------------------
  # reboot
  # lsmod |grep ipv6
  #
  # service dovecot start
  Starting Dovecot Imap: 
  Error: service(pop3-login): listen(::, 110) failed: Address already in use
  Error: service(pop3-login): listen(::, 995) failed: Address already in use
  Error: service(imap-login): listen(::, 143) failed: Address already in use
  Error: service(imap-login): listen(::, 993) failed: Address already in use
  Fatal: Failed to start listeners                    [FAILED]