The other thing you might need to know is that you can get different levels of verbosity when you run the journalctl command as root vs. non-root. When you "only" got the Starting and Finished messages, I'm betting you ran the journalctl command as a non-root user. Try it as root.
Which could be dangerous as it seems as usual systemd does not follow best practices and conducts these searches of potentially untrusted input as root even if it escapes etc. it is also written in C and not something safer like Ada.
If you want all of the "mail-related" messages to be in a single file, install rsyslog and use the traditional human-readable log files under /var/log.
Then you can use permissions to give a user safer access to analyse them. -- Regards, Kc
