Restrictions on recipient addresses
smtpd_recipient_restrictions

This parameter specifies restrictions on recipient addresses that SMTP clients can send in RCPT TO commands.

These destinations do not need to be listed in $relay_domains.

The following restrictions are available (* is part of default setting):

Restrictions are applied in the order as specified; the first restriction that matches wins.

Specify a list of restrictions, separated by commas and/or whitespace. Continue long lines by starting the next line with whitespace.

NOTE: YOU MUST SPECIFY AT LEAST ONE OF THE FOLLOWING RESTRICTIONS OTHERWISE POSTFIX REFUSES TO RECEIVE MAIL:
reject, check_relay_domains, reject_unauth_destination