Mastodon doesn't like Addy aliases
It appears to be an unauthorized provider.
Addy aliases are temporary email addresses (aliases) that forward messages to their primary email account. This can help protect a user's real email address from spam, unwanted marketing, or potential data breaches.
Get a domain and link it so you have your own domain addy.io . It's not very hard.
You can use the domain for other things too.
That defeats the purpose.
How does it defeat the purpose?
The domains have nothing that is traceable to you. addy.io provide that opportunity for you to do just that. What's the issue?
If you have your own domain how are they going to trace that to anyone else?