Our search is built on top of the Xapian search engine. We blogged about the underlying tech a while ago. You can sign up a free trial and have a play pretty easily.
Search works decently, but they index the whole message, including quoted text. So a search term shows up in the original message, as well as all the replies downthread.
Yeah, we're working on identifying whether something is in quoted or non-quoted text. That one is quite tricky to get 100% right, so we err on the side of matching more messages.
https://fastmail.blog/2014/12/01/email-search-system/