In /usr/local/etc/postfix/access_sender, add email or domain which you want to block.
To reject and send explanation message, that mail server wont accept the mail, you must use reject_unverified_sender not REJECT. REJECT rejecting the mail without explanation message.
after add desirable domain or address, you need to update access sender database:
– YOU MUST BE IN THE DIRECTORY WHERE IS ACCESS SENDER FILE
If you don`t have in main.cf strings about error messages you must add them on hand.
unknown_local_recipient_reject_code = 550
unknown_client_reject_code = 550
unknown_address_reject_code = 550
unknown_hostname_reject_code = 550
unverified_sender_reject_code = 550
unverified_recipient_reject_code = 550