Old emails reappear in Inbox

[widow]

Okay, I have this very strange thing going on.

A couple of months ago, I had a user complaining that old emails from months ago, reappeared in his inbox. As a read email, with the original date when the mail was delivered a couple of months ago. I didn't have any clue what could cause old emails to come back in his inbox, so I changed his password to make sure nobody was messing with his email account. After that, it went OK for a couple of months.

Yesterday the same user came to me, complaining that a lot of old emails reappeared in his inbox again.

What can cause this problem? The user is only using the webbased zimbra client. We always update our mailserver when there's an update available. Is there perhaps a logfile where I might be able to see where those messages came from?

[firemike]

When a user includes an external mail account into his zimbra-account by fetchmail / pop - it may be, that the original external mail is not deletet!
Several configuration changes can then cause the old external mails to be picked up again.