Java - what to do when the launcher refuses to start because Java is missing or too old, and how to adjust the Java memory allocation.
Connection problems - steps to diagnose and fix issues when the web admin UI is unreachable in the browser.
Zimbra configuration - how to verify the milter is correctly registered in the Zimbra MTA and test the mail flow.
Services - how to diagnose a service that does not start using systemctl status and the journal.
Logs - reference for every file the milter and web services write to var/log: launcher output, the application log, worker pool statistics, message trace files and body dumps.
Directory service - verify the connection to Zimbra's LDAP: the zmpostfix bind account, base DN and an ldapsearch test.