For reference Yahoo are absolute shits when it comes to mail delivery. They literally will black hole addresses, domains and IP addresses with no recourse ever. The sending server will get a response code back of roughly "fuck you until you've manually filled in this form at this URL". The form is submitted by any attending mail admin if they even notice the response code, then silently with no confirmation they will either ignore you until the universe dies or suddenly mail will start working. If you fuck the form up, you have to wait 6 months to try again.
They actually stopped me hosting my own email because I couldn't be bothered to deal with this shit any more.