The old format was basically "concatenate all mail together in one giant file". I believe newer versions support Maildir ("pretend you're an IMAP server"), part

The old format was basically "concatenate all mail together in one giant file". I believe newer versions support Maildir ("pretend you're an IMAP server"), partly to support antivirus better (i.e. when things get quarantined under you, hope only the bad message gets moved instead of all your mail). They do have a weird database format ("mork"), but that is for state tracking, address books, etc. and not the actual messages, I believe.