Como eu estava indo em cada cenário, eu descobri!
/etc/postfix/main.cf
mynetworks = 127.0.0.0/8, 10.0.0.0/8
relay_domains = example.com, example.org
smtpd_recipient_restrictions = check_recipient_access hash:/etc/postfix/sender_access,reject_unknown_sender_domain,reject_unknown_recipient_domain,permit
smtpd_relay_restrictions = permit_mynetworks, permit_sasl_authenticated, defer_unauth_destination
smtpd_sender_restrictions = check_sender_access hash:/etc/postfix/sender_access,check_recipient_access hash:/etc/postfix/sender_access,reject_unknown_sender_domain,reject_unknown_recipient_domain,reject
/ etc / postfix / sender_access
example.com OK
example.org OK