Append SMTP headers

(optional) If specified, appends this text to the end of the message header of every mailing list message. A message header text has the form of Key: Value.

If you want to add text to the header that is not a standard email header, you should prefix your key name with X- so that it does not interfere with normal operation of electronic mail. For example:

X-Legal: Contents Copyright 1997 Acme Corp Ltd.