On ubuntu 16.04 LTS that just have a ip4 and no ip6 protocol enabled, the dovecot install crash when trying to install dovecot-managesieved.
It crash Iredmail install. It is not related to Iredmail, but on dovecot.
If IPV4 is only enabled, please add into dovecot-core configuration file into /etc/dovecot/dovecot.conf at first line
listen = *
It will fix the install issue.