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When you send an email to an address that doesn’t exist or to a server that is having trouble delivering it, you get a Non-Delivery Report back. Spammers have been exploiting these bounce messages for a while now, as a way to get around spam-filtering measures. However, last month saw NDR spam hit an all time high with 20% of all spam messages using the trick. That’s a rise, according to security specialists PandaLabs, of no less than 2000% when compared to the number of different NDR spam samples seen between January and June this year.
It is a clever technique, and obviously one that works or the spammers would waste their time and money exploiting it. The point being that the bounce messages themselves are more often than not genuine, with the server function being exploited to distribute the spam (sent as an attachment to the bounce notice) using the sender’s real name.