I pair fastmail with postbox on mac, and search is terrific (because of postbox). Quite expensive though.

Also nothing as good on mobile. But fastmail search has been okay, I'm not too handicapped.

Search seems adequate for my needs. And it works great with any IMAP email client.

Actually the strangest thing I find amazing, is FastMail's endless scrolling being basically magical. I can scroll through over 2500 messages in a folder of mine, top to bottom, instantly. It's not paginating, it's not locking up at the bottom of the first 100 to stop and load more. It :just works: in their UI.