event registration problems

In our indico instance when our teachers or students try to register to an event in the email field they always get the error (The domain used in the email address does not exist.) We have version 3.1.1. Any other email address works.
Another particularity is that if it is a user already registered in indico in the email validation works and identifies the user, but when submitting the registration in the event I get the error (Email Address: Invalid email address).
What could be the problem?
Thanks for your attention.

Can you share the domain part after the @ for which it fails? Privately in a DM if you prefer.