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Backscatter (bounces to emails you never sent) are an annoying problem due to spammers abusing the way internet email was originally designed. Unfortunately most backscatter was not correctly detected by the existing spam filter. Fortunately, we now have a new more advanced system for detecting and stopping these backscatter emails.
To enable this improved protection, just go to Options -> Spam / Virus protection. If you already have Normal or Aggressive level protection enabled, just click Done, this will enable the new protection. If you have Custom settings, select what you would like to do with backscatter from the Backscatter Action pop up menu, then click Done
For more information on what backscatter is, how it occurs, and how the new proection system works, see this FAQ entry.