I have a user who had this problem not that long ago:
http://community.spiceworks.com/topic/320038-lotus-traveler-push-message-error
I managed to resolve this issue after a call to IBM, but now the user occasionally appears to send emails as Administrator@ourdomain.com. I can't find a reason for this, and no pattern has emerged yet (although I'm monitioring it). His permissions etc are the same as anyone else's, and this doesn't happen all the time.
Any suggestions on what to check?