"Bounce" messages are a type of automatic responses sent by a program or server in reply to an email that has been received.
Please note that there are many types of auto replies used in SMTP conversation and a bounce message is a type of auto reply.
These messages are sent in accordance with RFC guidelines which require Mail Transfer Agents (MTAs) to inform the originator once it has accepted an email for relaying or forwarding purposes.
In SMTP, bounce messages are sent using a null sender address: <>. They also typically include a From: Mailer-Daemon on the recipient side.
The main purpose of bounce messages are to enable administrators to spot problems and give enough information to facilitate a fix.
Parts of a Bounce Message
Bounce messages usually contain information that assists the original sender in determining why the message was not delivered.
The common elements are outlined below.
Last-Attempt-Date: Thu, 22 Jun 2017 14:12:36 -0700 (PDT)
Diagnostic-Code: smtp; 553-Sorry, your email address [email protected] has
Delivered-To: [email protected]