W32/Mytob.be@mm is a mass-mailing worm that combines W32/Mydoom@MM functionality with W32/Sdbot.worm functionality. The virus arrives in an email message as follows:

From: (Spoofed email sender)
Do not assume that the apparent sender address is an indication that the sender is infected. Additionally you may receive alert messages from a mail server saying that you are infected, which may not be the case.

More information can be found at this McAfee page.