Then you use one of their alternate domains. Or you point a domain to one of their alternates. Check out the MX record for meowcats.fun, for example. It points

Then you use one of their alternate domains. Or you point a domain to one of their alternates. Check out the MX record for meowcats.fun, for example. It points to suremail.info, which points to mailinator. I have never had that domain rejected by a form. Shhh, don't tell anyone ;)