Thunderbird is working on enabling exchange, and meanwhile you can combine it with TBSync plus its provider for exchange AcriveSync extensions. And given TB hadn’t care so far about tray, to at least avoid TB dying by mistake, you can also add Minimize on Close extension. Mail would still be IMap, so it’ll work as long as the outlook provider enables IMap support, but for the company I work it’s enabled. But such support is coming up on TB. Not sure if its solution would be 100% open source, but I hope it is, otherwise, I’m not sure if everyone will want to have a blob proprietary binary inside TB…
betterbird tray solution doesn’t work on wayland, given a bug on common code (affects both, Firefox, Thunderbird and derivatives). Just in case that’s one of the motivations of using betterbird. That by the way was the only feature that really made me look at betterbird, and as it didn’t work, I went back to TB. And if you’re wondering, birdtray doesn’t work on wayland, 😑.