Bounce Messages, in short are automated responses from a sending MTA to the SMTP sender of the message informing them of any errors in the delivery of that message.  There are two types of bounces:

- Hard (Permanent) Bounce, which is denoted with a 5xx SMTP Diagnostic Code
- Soft (Transient) Bounce, which is denoted with a 4xx SMTP Diagnostic Code

In addition to the SMTP Diagnostic Code, an SMTP Extension code and SMTP text message are also typically associated with the bounce message.  The combination of all the pieces should give the receiver of the bounce sufficient information to determine the root cause of the bounce message.

Senders should exercise proper bounce management by analyzing all applicable data points in the bounce message to determine the proper action that is needed.  For example, it is recommended that senders promptly remove addresses returned as permanent errors by an ISP or other receiver after analyzing the SMTP Code/Extension/Text Message.  The reputation of senders who continually mail to permanently inaccurate addresses (hard bounces) should be negatively impacted if the addresses are not removed and/or source of the problem not remedied. Receivers are encouraged to provide accurate and informative error codes, such as a 5.7.1 error code for a policy violation.

As a note, although RFC standard exists to define bounces and errors in the SMTP transaction, it is common for receiving systems to create their own policy for bounces and thus not follow what is defined in the RFC.

Detailed information as defined by RFC can be found at and  In addition, it is strongly recommended to review ISP postmaster sites to review their policies and bounce definitions.