I love Thunderbird. When it went out of active dev, I switched to Postbox, assuming that they'd really get to work on the client. They didn't. They made it look mildly prettier, but stripped away a few of the good features without adding anything of value (to me, at least.)

I spent some time hopping around clients and eventually landed on plain old MacOS Mail with an extension to put the unread count in the menubar. It wasn't perfect, but it was good enough.

On a side note, I don't understand why clients don't all allow plaintext composition and a menubar inbox count.

Anyway, it's great to see TBird back in active development. It's consistently been the best email client for my needs over the years.