postconf -d smtpd_recipient_restrictions
:% smtpd_recipient_restrictions = permit_mynetworks, reject_unauth_destination
postconf -d mynetworks
:% mynetworks = 127.0.0.0/8 <serversSubnet>
pode querer:
smtpd_recipient_restrictions = permit_mynetworks permit_sasl_authenticated reject_unauth_destination
ou mynetworks = 127.0.0.0/8 <serversSubnet> <clientSubnet>
talvez ambos