what causes spurious email virus rejections?

[dmorin at lear.morinfamily.com (Duane Morin)]

Once in a blue moon either I or my wife will get an unexpected email that 
says "Could not deliver message to <unknown address X> because it 
contained a virus."  Never heard of that address, never intended to send 
anything to that address.  I'm scanning regularly for viruses.  And I'm 
also thinking that if I really did have a virus then wouldn't I see more 
of these messages?  What's it mean when I see one, oh, i dunno...every 
couple of months?  Should I just ignore it?

Duane