I have always used a full-on mailserver with virtual mailboxes as used by ISPs and fortunately I found a ready made ansible script to do the job.
Fortunately this script is written for Ansible 1.9 and Debian Jessie so I don't envisage many problems.
I followed the README and checked the scripts. Variables have to be supplied to fit each use case, found in group_vars/all. For the database passwords I used my previously obtained unique value: