When an email bounce happens after completing the Simple Mail Transfer Protocol (SMTP) and subsequently returned to the sender, such a bounce is an asynchronous bounce or an async bounce. It is the direct opposite of synchronous bounce.
If Server 1 sends an email to Server 2, then Server 2 accepts the email after which it ends the connection to Server 2. Sever 2 may later discover that the email address is unrecognizable, prompting a bounce message to Server A.
This is an asynchronous bounce – it is asynchronous because the bounce is not synchronized with the delivery of the initial message. Asynchronous bounce may come at any time after a perceived successful delivery, even if it is weeks later.
Asynchronous bounces present a couple of operational problems. One of these is the backscatter. The mail server from which the asynchronous bounce originates sends it to the email address in the bouncing email’s “envelope from” / “bounce address.” If the mail being bounced is spam, the email address is most likely a fake address or the address of an innocent third party.
Backscatter bounces are more rampant than we think. Doing proper filtering and similar delivery decision-making moves with the sending mail server still connected is recommended. This will help to safely reject the mail. Sending an asynchronous bounce should be a last resort, especially when you discover that the message is unable to deliver.