I am running Exchange 5.5 on NT 4.0. I recently noticed that whenever a mail is sent on this mail server to any email address that consists of underscores and hyphens like this example (firstname_lastname@xxxxxxxxxxxxxxx), the recipient does not receive it, and the mail server will not send back any warning notifications or what so ever like " A message that you sent could not be delivered to all of its recipients". Does anybody has an idea where to look? Thanks john