set personal-name

SET PERSONAL-NAME [text]  -  define your name, for the FROM field

The PERSONAL-NAME variable defines the personal-name part of the FROM field
  in your outgoing mail.  Unlike the DEFAULT-FROM variable, it leaves your 
  user ID (in <> brackets) intact, and does not generate SENDER and REPLY-TO
  fields.  If the variable is not defined, your name for the FROM field 
  comes from the file /etc/passwd.

Punctuation marks in personal-name may cause mail problems in replies to you.
  If you need to use punctuation, put the personal-name in " quotes.

To unset it, type SET PERSONAL-NAME with nothing following.

Example:
  If  user abc, Aethelred B Cooper, does this:
    MM>set personal-name Al Cooper
  Then the FROM field will show:
    From: Al Cooper 

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