only one account can write to the other (fwd)

Hi: I have made an alias at /etc/aliases which manages a list to which a I have to write every once in the while, the thing is that I only want that account a@mydomain.com be the only one to be able to write to b@mydomain.com I use sendmail. Thanks, Raúl _______________________ Raul Gutierrez Segales oxford@campoalto.edu.py