Zimbra stores messages as simple UNIX files in the filesystem, 1 message per file. They're grouped by mailbox, so all of a user's mail is stored under
/opt/zimbra/store/0/<mailbox_id>/msg.
Note that this is not enough to guarantee SOX compliance, as deleted messages are purged from the store. Please vote for
the archiving RFE if you'd like to see a real archiving feature.