System Administrator: Undeliverable...

Backscatter (also known as outscatter, misdirected bounces, blowback or collateral spam) is a side-effect of e-mail spam, viruses and worms, where email servers receiving spam and other mail send bounce messages to an innocent party. This occurs because the original message's envelope sender is forged to contain the e-mail address of the victim. A very large proportion of such e-mail is sent with a forged From: header, matching the envelope sender.

When a spammer uses your email address to spam, all of the undeliverable mail gets returned to YOU. So instead of getting the odd spam msg once in a while addressed to you, you will get hundreds of spam email 'returned' to you because the other system thinks you are the sender. Spam filters don't usually stop these because they are coming from a valid sender, the mail system administrator to wherever it was originally sent. In other words, this particular kind of spam is a major pain in the arse. I've heard of this phenomenon but had not experienced it until today. The first couple started trickling in this morning at 9:28am. By 9:58 (30 minutes later) I had received 513 such messages.
Spending my Friday (the 13th) deleting spam from my inbox, NICE!