[Answer]-Django admin search for non existing returns all items

(0.133) is the query looking for the email you mentioned.
(1.593) is selecting all users.

Either the first is returning all results, in which case one of mail, uid or login is in fact holding verylongemailproble@gmail.com as a substring for every record.
Or FrontendUserAdmin is returning every record for some other reason.

In the second case, maybe django-debug-toolbar will show where this query was ran inside django, that will help you dig through the code.

Investigate whichever sounds more likely or easier to you.