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.
It’s all about the instance. For the Lemmy instance I chose I had to write a few sentences. For mastodon I had to provide a picture of my SubGenius pastor card. I intentionally picked ones that had requirements.