If I got your question right, You need to modify this file for this --> /etc/mail/access
See these examples.
Connect:mail.yahoo.com REJECT # Allow yahoo.com domain
To:email@example.com REJECT # Reject a1 user from recieving mails
From:firstname.lastname@example.org REJECT # Reject b1 user from sending mails
A restart of sendmail is required after making changes.