Debugging try to log as much as possible. There are couple places where you can find valuable logs when debugging:

  1. docker logs --tail=100 mailserver
    there should be all information about starting and stopping services.
  2. /your-data-dir/logs
    its essentially copy of /var/log from inner container. There are couple special directories besides standard ones:
    • admin-app - web administration errors log
    • delivery - all logs from email delivery should be logged there
    • dmarc - received DMARC reports
    • s6 - contains output from supervised services
  3. /your-data-dir/domains/
    whenever sieve script fails there should be error logged to this file under user mailbox directory

PRO version

In PRO version you can search through delivery logs at web administration.