Right now, as far as I can tell and from what I've seen in old issues and current jira issue, searches only search for primary email address. Worried about performance issues.
So it appears there is no way to search any email address that isn't marked as the primary other than writing sql or a custom search.
Maybe I'm missing something and there is a way. If not, to me this is major weakness, but maybe I'm alone in my thinking. The issue has been out there for 6 years and was last put off until after 4.7 so maybe it isn't a big deal for most people.