What is an email double bounce?

Background Information

A double bounce occurs when a bounce cannot be delivered. E.g. An undeliverable message is accepted and a bounce is generated. The bounce itself is sent to a non-existent domain. The bounce will be attempted for some interval but at some point the Mail transfer agent (MTA) will identify it as not deliverable. Then the bounce is bounced creating a "double bounce." The double bounce does not generate another email because the bounce envelope sender was <> so there is no address to which to send the double bounce. Double bounce action simply eliminates the bounce message that was queued up. (Analogy: where do you return the "return to sender" postal mail if there is no valid return address? You cannot, so after you figure that out, you just discard it.)

The RCPT TO address of a bounce message is the Envelope Sender (aka MAIL FROM) of the original message. The bounce RCPT TO is not generated from

A double bounce occurs when a bounce cannot be delivered. E.g. An undeliverable message is accepteope sender (<>) so it cannot be returned to sender.