Thunderbird

Thunderbird takes as many accounts as you care to add — IMAP, POP, Gmail over OAuth, Exchange via EWS — and gives each one a real address book, a calendar, filters that run locally as well as server-side, and OpenPGP that is part of the compose window rather than bolted on beside it. It is the client to use when you want your mail sitting in files on your own disk with nobody between you and the server.

What the money buys is unusual, because there is no money. There is no free plan to outgrow, no seat count, no AI add-on quietly metered against you; MZLA funds the work through donations and the macOS build comes straight off thunderbird.net.

The cost turns up elsewhere. This is a cross-platform application and it behaves like one. The calendar and contacts feel like neighbours rather than parts of a whole, the keyboard model is its own, and a mailbox with a few hundred thousand messages will drag in ways a native client would not. MZLA links a separate hosted service from the homepage, which is worth not confusing with this.

If you want a Mac app that feels designed for the Mac, buy one of the paid clients. If you want mail that will never present you with a renewal notice, this is the floor — and it is a surprisingly high floor.