Backscatter (e-mail)

Backscatter (also known as outscatter, misdirected bounces, blowback or collateral spam) is incorrectly automated bounce messages sent by mail servers, typically as a side effect of incoming spam.

Recipients of such messages see them as a form of unsolicited bulk email or spam, because they were not solicited by the recipients, are substantially similar to each other, and are delivered in bulk quantities. Systems that generate email backscatter may be listed on various email blacklists and may be in violation of internet service providers' Terms of Service.

Backscatter occurs because worms and spam messages often forge their sender addresses. Instead of simply rejecting a spam message, a misconfigured mail server sends a bounce message to such a forged address. This normally happens when a mail server is configured to relay a message to an after-queue processing step, for example, an antivirus scan or spam check, which then fails, and at the time the antivirus scan or spam check is done, the client already has disconnected. In those cases, it is normally not possible to reject the SMTP transaction, since a client would time out while waiting for the antivirus scan or spam check to finish. The best thing to do in this case, is to silently drop the message, rather than risk creating backscatter.

Measures to reduce the problem include avoiding the need for a bounce message by doing most rejections at the initial SMTP connection stage; and for other cases, sending bounce messages only to addresses which can be reliably judged not to have been forged, and in those cases the sender cannot be verified, thus ignoring the message (i.e., dropping it).

Authors of spam and viruses wish to make their messages appear to originate from a legitimate source to fool recipients into opening the message, so they often use web-crawling software to scan usenet postings, message boards, and web pages for legitimate email addresses.

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