Ordinarily, errors are bounced to the envelope sender. The Errors-To: header specifies the address, or addresses, to which sendmail should send additional notification of delivery errors.

The Errors-To: header is intended for use by mailing lists to prevent errors in a list from being rebroadcast to the list as a whole. For example, consider the mailing list allusers. Mail that is sent to this list should contain the following header lines:

To: allusers
From: allusers-submit
Errors-To: allusers-errors

The From: header allows reply mail to be submitted for distribution to the list. The Errors-To: header causes error notification to be sent to allusers-errors so that the maintainer can fix any errors in the list. The original sender also gets error notification unless the mailing list software represents the maintainer in the envelope (Section 13.5.1).

Under SunOS and V8 sendmail the Errors-To: header is flagged in conf.c with the H_ERRORSTO header flag (Section 25.6.10). This allows other headers to be declared in that file as error redirect headers. Under pre-V8 SunOS sendmail the Errors-To: header is ignored if the error mode set by the ErrorMode option is m (See this section).

Under V8 sendmail the Errors-To: header is ignored unless the UseErrorsTo option (UseErrorsTo) is true. It does this because the Errors-To: header violates RFC1123. Errors-To: was needed only to take the place of the envelope sender in the days when most Unix delivery agents couldn't differentiate between header and envelope.

The Errors-To: header should never be declared in the configuration file.