To trak this down:
1) If you send an email to this domain, wich lines appear in the mail log?
2) Is the domain listed in the file /etc/postfix/local-host-names ?
3) Is the email address that shall receive the emails listed in /etc/postfix/virtusertable ?
4) Which output do you get when you exectute
dig MX mydomain.com
Replace mydomain with your domainname