Corrupted Mailboxes

[Sioril]

I'm running a Tiger server for mail. With a around a couple dozen users. A while ago a users mailbox couldn't access mail via pop or imap. Both enabled on his account and the server. I thought that maybe it was because he had a few thousand e-mails in there, cleared a few and rebuilt his mailbox. Than it happened again more recently, to him, and than another user as well. I did the same thing to these ones, and it seemed to be working fine, now again today the same users mailbox is having the same problem. Anyone have any idea what is causing these problems, how it can be prevented etc? To rebuild the mailboxes I was using the Cyrus Reconstruct -r thing in the terminal.

Oh yeah, I forgot to mention the last time I repaired this box was two days ago.