Having Issues with adding certain email addresses to google play beta test list?

I am having some issues regarding adding certain email types to our google play closed testing list.

Official google documentation says that the accounts added must be @gmail, however, we have been able to add emails from users like @talktalk.net and our own company email @cufflink.io recently we had a user request to join our beta programme with an email that ends with .co.uk and we get the following message:

Google play error message

Can anyone shed some light on this? what is allowed and isn't allowed as going forward we will need to inform users what they can and cannot sign up with

Any help will be much appreciated