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Several header lines have special interpretations defined by the configuration file. Others have interpretations built into sendmail that cannot be changed. These built-ins are described here.
If errors occur anywhere during processing, this header causes error messages to go to the listed addresses rather than to the sender. This is intended for mailing lists.
The ``Errors-To:'' header was created when UUCP did not understand the distinction between an envelope and a header; this was a work-around that provided what should now be passed as the envelope sender address. It should go away. It is only used if the UseErrorsTo (l) option is set.
RFC 822 requires at least one recipient field (To:, Cc:, or Bcc: line) in every message. If a message comes in with no recipients listed in the message then sendmail will adjust the header based on the ``NoRecipientAction'' option. One of the possible actions is to add an ``Apparently-To:'' header line for any recipients it is aware of.
The Apparently-To: header is non-standard and discouraged.
The Precedence: header can be used as a crude control of message priority. It tweaks the sort order in the queue and can be configured to change the message timeout values. The precedence of a message also controls how delivery status notifications (DSNs) are processed for that message.