Unread mail becomes read after replying on other mail

We have the following problem on 2 clients:When the user has 2 unread mails in his mailbox, and he replies (or forward) on one mail, the second mail becomes also ‘read’.

Any idea what the problem is ?

Subject: Have you verified that the user is correct?

Have you seen with your own eyes the un-forwarded email getting marked as read? If not, then the likely culprit is the user mis-reporting the real behavior, which is probably that only the actually-forwarded email gets marked, because this database property isn’t set on their mailfile:

Then again, it’s always a small possibility that the user is not mistaken – in which case I have no idea beyond the standard design replace/compact route.