This bug is almost a year old and still unfixed. Worse than that I did not get a single comment on the issue. Is this package dead?
When fixing it you should also fix this: Aug 1 12:34:56 hostname systemd[1]: /lib/systemd/system/dovecot.service:9: PIDFile= references path below legacy directory /var/run/, updating /var/run/dovecot/master.pid → /run/dovecot/master.pid; please update the unit file accordingly.
