RE: replication and backup

• NOT expunging the emails that have been marked as deleted on the slave server or expunging only manually

Yes I think it is depending on your storage format (mdbox only?). I am running "doveadm purge -u" every two months. If I would make a new server I would also use the LAYOUT=index this does not give you issues with mailbox names, however there were some other issues, but I think new versions of dovecot are ok.