set default-from

SET DEFAULT-FROM [text string]  -   define a FROM field for all mail

If nothing is defined, the FROM field contains your name and userid 
  (address) as shown in the /etc/passwd file.  

Possibly, you want to change your name in the FROM field, and leave 
  the userid (address) as it is.  See SET PERSONAL-NAME to do that.

SET DEFAULT-FROM changes the entire FROM field in all messages you send
  to the text you provide.  However, fields labelled SENDER and REPLY-TO 
  will then appear, showing your name and userid as in /etc/passwd.  You
  can erase or modify the REPLY-TO field but not the SENDER field.  Setting
  DEFAULT-FROM is the same as using the FROM command with the same text
  every time you send a message.

To unset, type SET DEFAULT-FROM with nothing following.  

For more help, type "help" and one of the following:
(at any prompt:)  save-init   set   show   set personal-name   signature-file
(at the Send> prompt:)  from