We get this a lot from our sites too.
The latest batch of viruses send spam messages out pretending to be from your domain. Hopefully those that get these stupid messages will see them for what they are and just delete them. Even better, they'll be behind some good protection.
The best one I've seen lately was from ebay - "Your account has just been debited £xxxx because you have just bought a 12" supa-dupa vibrator. Please click on the link to confirm payment." My boss went spare when he got that one... Unfortunately, he'd already clicked the link and got the appropriate NAV warning.