Users receive multiple copies of the same internet message

Since june 24, users receive multiple copies of the same internet
message. It seems to affect many of our users. Only External email are
resent. In my case, I receive the 5 same messages every 10 minutes. The
header show the message was receive between 01h00 and 07h00 on june 24.
Where can I delete these messages? I rebooted the server yesterday, but
the messages still resent. I stopped MTA to see if it's not an external
email server that was sending the message over and over, but I received
the 5 emails when the MTA was closed.

Any idea?

GroupWise 5.5epSP5 (MTA + POA) running on Netware 6.5sp1
GroupWise 5.5epSP5 (MTA + GWIA) running on Netware 5.1sp6

Thank you