setup mercury mail server for website devement

may I setup mercury forward all email in and out to one mailbox for the test. example if PHP sends mail to aaaa@gmail.com. I need to send to postmaster@localhost, not aaaa@gmail.com if PHP sends an email to staff@localhost bcc bbbb@gmail.com. mercury send it to postmaster@localhost, not aaaa@gmail.com

and if bbbb@gmail.com send email to someone in my domain. it moves in the inbox of postmaster@localhost

thank you.