[Exim] Outlook Express fails to delete messages

I have a user with Outlook Express 6 who is having a weird problem.
Until last week he was using CommuniGate with no trouble. I switched
him to Exim and now, when he marks messages for deletion they show up
again as new messages after a few minutes.

This problem does not happen if he uses Mozilla. Also, closing OE
causes the messages to stay away.

Any clue what might cause a problem like this? This is a weird
problem.

Thanks,
Mark Lehrer