You can see the elasticsearch query if you expand advanced in the UI.
Because of the way that we tokenize the search string, it is likely that a query like email@example.com will match more than just the user with the email address you are entering.
If you want to match only the email address in the UI, the easiest way to do it is to preface the query with email:. So email:firstname.lastname@example.org.
Hope that helps.