The mailer was sending files out using the temporary URL with BlueHost, once the domain was moved over it was not sending mail over the domain; since mail went out through the main handler the issue is host related (as UBB.threads was able to send mail out on the temporary URL). Your host will have access to mail spooling to read any logs that're processed through their system that your CPanel user account does not have access to.
It could also be something related to DNS propagation at the time (which in this case should have sorted itself out), or it could be that a DKIM or SPF record on the domain is incorrect, your domain may be in a in a blacklist, etc.
