Hi tamzx and Anthony Cartmell,
Thanks for posting your issue here in Microsoft Community. Sorry for not being able to respond sooner. I understand that emails from your custom domain to Microsoft Account are being blocked and I'm glad to help.
@tamzx, I know that you have already tried to submit a Sender Information for Outlook.com Delivery e-form but I encourage you to fill out and submit another e-form. We apologize that you did not receive a response to your first request.
@Anthony Cartmell, please fill out the e-form found on this link to request for your domain to be removed from the block list. Please visit the link below to find out the reasons why the emails that you send to Microsoft Account are being rejected.
Why are the emails sent to Microsoft Account rejected for policy reasons?
You can find more information about Outlook Postmaster on the link below.
Outlook Postmaster
I hope this helps. Let us know if you need further assistance