Several header lines have special interpretations defined by the configuration file. Others have interpretations built into sendmail(1M) that cannot be changed without changing the code. These builtins are described in this topic.
Errors-To:
If errors occur anywhere during processing, this header will cause error messages to go to the listed addresses. This is intended for mailing lists.
The Errors-To: header was created when UUCP didn't
understand the distinction between an envelope and a header; this
was a solution to provide what should now be passed as the envelope
sender address. It is only used if the UseErrorsTo option
is set.
The Errors-To: header is officially deprecated and should
be avoided.
Apparently-To:
RFC822 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. This is not put in as a standard recipient line to warn
any recipients that the list is not complete.
The Apparently-To: header is non-standard and is deprecated.
Precedence:
The Precedence: header can be used as a crude control of
message priority. It modifies the sort order in the queue and can be
configured to change the message timeout values.