Trc4 what's your use case?
I don't think there's a built-in solution, because all the search filters included in Flarum core can be used by any user and having an email search would be a privacy nightmare.
In the context of a private forum or where search is restricted to admins only, my extension flamarkt/identity indexes the email into the Scout full text search.
A more flexible solution for a regular forum would be to create an email search gambit, protected by a dedicated permission. It's not a complicated piece of code, but I don't think we have an example ready.
If this is just a one-time search, you can connect to your database and filter the users table on the email column.
If you're trying to ban specific emails, you can use this extension https://discuss.flarum.org/d/28377-email-whitelist It only applies to new registrations though.