set aliases-use-groups

SET ALIASES-USE-GROUPS NO/yes  -  whether to show mail-aliases in sent mail

With the usual NO setting, if you use a mail-alias when you type in an
  address, it is changed in the outgoing mail to show the real addresses
  (the contents of your mail-alias).  If the mail-alias was for a list of 
  addresses, the people receiving your mail will see who else got the 
  message, and be able to send replies (or regular mail) to the other people.

If you change it to YES, the people receiving your mail will see only your
  mail-alias in the address (identifiable by the colon and semicolon :; 
  after it).  It might be better to show only the mail-alias if it is a 
  long mailing list that would appear as a very long header field.

For more help, type "help" and one of the following:
(at any prompt:)  save-init   set   show   set mail-aliases   define   who