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 depends on the instance. I created an account on mastodon with guerillamail which would be even more suspicious for an average website than your alias email. A tip, if you do not mind using icloud email, you can even create one with some effort without giving real personal data and using apple hardware (Virtual machines ftw), you can take advantage of its free alias email addresses which would be accepted even on most strict websites.