This is a true alert.
Any email with "Mail Server Report" in the subject line is a new mass mailing worm and should not be opened . It comes with an attachment in zip format. Apparently there is a Zip file attached so be very weary of any thing with a zip file on it.
The message tells you that a worm was detected in an e-mail that you sent out and asks you to open and install the attachment to fix the problem. Only, it ISthe problem! DO NOT DO IT!Delete the e-mail.
This is a real threat.
You can check it out at snopes: http://www.snopes.com/computer/virus/mailserver.asp