Like paper mail sent via the post office, undeliverable emails typically return to the senders with messages explaining why delivery failed. Reasons for non-delivery include an unknown user, a disabled account, a full mailbox and failed sender authorization. Reasons for not returning the email or a non-delivery notification to the sender involve the detection of spam, viruses, phishing attempts and other hostile content.
The Internet Engineering Task Force, or IETF, publishes formal documents concerning Internet protocols and other related topics. RFC 5321 is such a document concerning the Simple Mail Transfer Protocol, or SMTP. Drafted by committee and reviewed by all interested parties, this Request for Comment defines how electronic mail progresses from its originator to its recipient. It stipulates that an email server that accepts a message it can't pass on must return the message along with a reason why delivery failed to its originator.
Mail agents produce such messages, also known as bounce messages, for a large number of reasons. A bounce message typically contains information to aid the sender in understanding why delivery failed, including when the messaged bounced, which email server bounced it, the reason that server bounced it, the headers of the bounced message, and a portion or all of the original message.
In certain circumstances, however, an email server should not send a bounce message to the originator but should instead drop it. For example, if an email server determines that the undelivered message is spam, it is highly likely that the "From" address is not actually the message originator. Collateral spam, often called backscatter or blowback, results from returning volumes of undeliverable spam to someone other than the original sender. It also is considered spam and can land the offending email server on a blacklist.