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.
schnurrito@discuss.tchncs.de 1 month ago
“Mastodon” is probably not doing that, a specific instance might be though?
SolarPunker@slrpnk.net 1 month ago
Yeah good point, in my case it was mastodon.social