Valid email is being blocked!

I have a customer with valid email who is not able to submit forms at all, 'cause GF is considering his domain as invalid (* The email address entered is invalid, please check the formatting (e.g. email@domain.com).)
Can you/the team please look into this? Perhaps there is an exception that can be added to the form for email addresses that leverage @wi.rr.com, but I did not find any documentation on how to do that.

Email field validation is actually performed by WordPress core, not Gravity Forms. Out of the box WP only performs a basic check to confirm the value looks like an email address. The WP function used does include a filter so it’s possible the theme or another plugin could be overriding the result.

Have you tried running through a conflict test?