Hi SDuddy,
The error message "smtp;550 5.7.509 Access denied, sending domain apps.myob.com does not pass DMARC verification and has a DMARC policy of reject" means that the recipient's email server is blocking emails from the domain apps.myob.com because it fails DMARC verification. This can happen if the recipient's email server is set to automatically reject new automated emails as a security measure.
Usually, resending the email can fix the issue. If it doesn't, the recipient's IT team should look into it. Also, make sure your network firewalls aren't blocking the emails.
Cheers,
Princess