Where indexing and searching mails is concerned, notmuch is the best I’ve seen. Do note that this is not an e-mail client, it only indexes, tags and searches (following the “UNIX philosophy” of doing one job well).
I personally use it with neomutt as a mail user agent, which is almost certainly not what you want. Notmuch supports other clients but they’re all pretty arcane.
So this is not a recommendation, just a glimpse into advanced e-mail setups I guess.
You don’t. Even if you’re happy to support the developers of the software you use (which is great!), I think it makes more sense to download and give it the spin first, then donate later.