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Please note, I'm not infected, and don't run any of the software that is the root, my firewall and windoze are both up to date, so I'm not talking about getting "attacked", but about the fallout of someone else's attack... Oh, and from the timestamps I can see they're sending out thousands more of these with other From: addresses...
Anyway - 18 hours, 2500 "bounced" emails and counting...
Thank god I'm not on dialup any more ;-P
Bounced emails could be caused by a virus. But with such a huge amount of messages bouncing back to you, I suspect it could be some SPAMMER using your adresses in it's forged headers.
It could also be someone using formail exploits from your sites. Did you ask your web hosts about recent suspicious activities on your sites?
What I really hate is that it's making up emails based on my hostnames - but I get every email address to them, so filtering is easy, just makes a *huge* list in the Spam folder ;-)