With this particualar error most of the time it has to do with antivirus software partucularly the email scanning either in or out usualy out since most email clients send first.

This most of the time occurs with improper configuration on the customers part, and occasionaly companies will do and update with will cause massive fun times for ISP's, but since this was affecting multiple AV programs (and I highly doubt that all of these companies came out with a patch the same day) I would agree with Luc that this probably was due to a feature that Microsoft implemented.

Oh and Faqt I still work for an ISP (but I don't do level1 or 2 tech support anymore ) but I did do the Tech1 thing for 3 years, and needless to say I feel do feel your pain