Have you noticed an increase in the number of suspicious "undeliverable mail" messages in your inbox? If yes, you are not alone in this predicament, and no, your server has not been hacked. There are reports that the amount of the so-called bounceback spam, or backscatter, has been rising globally.

Backscatter is the name for bounceback messages from legitimate e-mail servers that have been tricked by spammers. By using forged "From" headers, spammers make those servers to return system-generated emails to a number of recipients; being perfectly good servers, they are not on any spam blacklists and backscatter passes easily through spam filters. Often, the bounceback spam messages come with an attachment or an ad message.

Sounds like the perfect way to spam? Well, it isn't. There exist ways to secure e-mail servers against being exploited in a such a way, and every webmasters should make sure he has followed the accepted guidelines. Filtering incoming backscatter is discussed in this thread.