some mail do not go into mailbox and disappear

I have a user that used to recive mail from a specific mail address. a month ago something happend and
mails from that sender do not appear anymore in the users mailbox.
The mail is not in the Junk Mail and actually it is in the Trust List and u can see that it arrived to the mail box !
There are no Rules that do anithing to the mail. the mail appears in GroupWise logs as good mail that has been
recived and enterd the system.
Where can this mail go ? help..