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 Shelby Group Ltd.