I can confirm that, at least for us, the cause is the invoice link that AccountRight attaches to the invoices - to test this I forwarded the bounced back email from the sent box (copy and pasted the same email address that was used) twice, one unedited and the other with the invoice link table removed from the email. The unedited email bounced back again, the one with the table removed did not bounce back. Until MYOB lets us remove this link the only solution we can do on our end is to re-send the email with that link edited out and then it will go through.
I talked with support but they are sticking with their claim that this link can not be removed from the invoices even though it isn't there if you haven't gone through verification.