Oh, i'm so sorry that i misunderstood your issue...
Postfix complains "domain not found", it's caused by option "reject_unknown_sender_domain" OR "reject_unknown_recipient_domain" in parameter "smtpd_recipient_restrictions". Please show us output of below command:
# postconf smtpd_recipient_restrictionsIf it's caused by invalid sender domain, please try to move "reject_unknown_sender_domain" after "permit_mynetworks" (in smtpd_recipient_restrictions, too).