Allegedly, on or about 23 December 2018, Mike Chambers sent:
I thought /var/mail/ was the main root mail dir for users but I can't get dovecot not to error pointing to it, such as /var/mail/%u or whatever.
/var/spool/mail/username or /var/mail/username (one symlinks to the other, these days), is where the system delivers mail to that user.
When a user gets their mail, it may move into a /home/username/mail or /home/username/Maildir location. Perhaps all their mail may end up in their homespace, or just the messages that they move out of their inbox (it depends on how they configure things).