When a message is returned because of an error or because a return receipt was requested, V8 sendmail adds an Auto-Submitted: header. This header describes the reason for the return:

Auto-Submitted: auto-generated (reason)
Auto-Submitted: auto-replied (reason)     V8.12 and above 

The reason can be one of four things. It can be warning-timeout if the message has reached its Timeout.queuewarn option threshold (Timeout). It can be postmaster-warning if the failure was delivered to the postmaster as a result of a problem that the postmaster should fix, such as an MX configuration error. It can be return-receipt if the message was returned because of a Return-Receipt-To: header (Return-Receipt-To:) or a DSN NOTIFY=SUCCESS request (RFC1891). Finally, it can be failure for any other reason.

In all instances, sendmail also adds a Subject: header that contains a generic bounce message.

The Auto-Submitted: header should never be defined in the configuration file.