@carlux I have no way to check that, unfortunately
@smaurizi There is a way if you receive an email everytime someone follows you. You can compare those emails with your follower list. Of course, it's manual and time-consuming, but there it is.
Anyway, it probably has to do with instances that defederated with mastodon.social due to the Threads controversy.
@smaurizi Just FYI (you probably don't want to do it at this point), in Preferences, there's a section called Notifications. There you can select whether you want emails sent for different Mastodon events.
The UI may be different somehow (at qoto we use a fork and not mainline Mastodon), but it should be there somewhere.