Thundermail domain "@thundermail.com" not supported during registration

An individual from our Telegram community tried to join our new Fluxer community with a @thundermail.com email address, the email service from the venerable and open source Thunderbird desktop email client originally from Mozilla, and said they got a "Not supported domain" error. They did sign up from a different email address in the end, but I don't understand why Thundermail wouldn't be "supported". If it's intentionally blocked, would you please reconsider? It's not an email forwarding service like Firefox Relay or AnonAddy. Thundermail users are just as legitimate as Gmail users.