Backscatter (email)
Backscatter is the unwanted bounce or auto-reply traffic a mail server generates when it accepts a message and only later discovers, after the original sender's connection has already closed, that the message can't actually be delivered -- so it sends the bounce notification to whatever address the message claimed as its sender. When that claimed sender address was forged, which is common with spam and phishing sent under a spoofed-from address, the bounce lands in an innocent third party's inbox who never sent the original message at all. A domain can receive a flood of backscatter if spammers happen to forge it as the sender on messages that later bounce elsewhere, and receiving servers increasingly treat unsolicited bounce traffic itself as a spam signal. Validating the recipient during the SMTP conversation, before accepting the message, is what avoids generating backscatter in the first place.
By AdminKit team, Netbase JSC · Updated