Watchoutbelow,
I am also getting failure to send notices from large companies with Firewalls, saying it could not deliver mail (that I didn't send) due to a virus.
That is part of the SoBig Worm. I have seen it spoof emails from large companies. If you are worried you have the Worm or any other virus for that matter, try an online scan from Trend Micro:
Trend Micro's HouseCall
Take Care,
Richard