<--LDAP.
Not that it currently helps a lot like in an exhange environment where you usually use journalusers which the mailstore makes use of. and the synced users are synced before each archive task so the mails will end in their archives.
And it all depends of course on how you use the archive and which 'user' permissions are there.
Some people just archive w/o user-access.
Other delete/archive to keep the mailbox lean so users NEED access to the archive..