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Why am I receiving bounce-back notices for messages I did not send?

The undeliverable messages are coming back to you because spammers are using your email address as the FROM address in their messages. When mail is sent to an invalid email account, it is a standard practice that servers send back a non-deliverable notice to the address the message originated from. In this case, it appears the message originated from your email account, even though in reality it did not. This is happening to a lot of accounts right now.

From a Postini bulletin:

Spammers have increased their utilization of "spoofing" or "joe-jobbing" (falsifying the from address of an email) in the last few weeks, resulting in some email addresses receiving an increased amount of bounce messages for mail they did not send. Postini is currently capturing the majority of these messages when the bouncing server preserves the original spam content, but if the bouncing server has significantly changed the message then Postini may have allowed this message through as we do do not stop NDR bounces.