set header-options-file

SET HEADER-OPTIONS-FILE filename  -  define a file of extra header fields
Usually:                /dev/null

The HEADER-OPTIONS-FILE variable defines a file containing extra header
  fields to be placed in all outgoing messages.  

The file must contain header fields in the format "label: data", that is, 
  a single word (a string with no spaces), a colon, a space, and then 
  whatever text you like.  Each header field must be one line. 

To have no header-options-file, use the special filename /dev/null,
  which is the standard empty file.

Example:
    MM>set header-options-file ~/.mmheaders
  Your file .mmheaders contains a line like:
    Office: 800 Watson
  Now your outgoing mail will show your office address.

For more help, type "help" and one of the following:
(at any prompt:)  save-init   set   show   set user-header
(at the Send> prompt:)  user-header