It's so long since I've switched that I'm not sure anymore, but Postbox tried to be more Mac-like and had native integration with macOS Contacts instead of separate address books. I think it had some filter rules Thunderbird didn't have, and faster indexed search.

But now, I use the keyboard driven Tagging & Quick Move features [1] a lot. They work a bit like Alfred [2]. To file a message in my Filtered folder, I type "v fil Enter" and by the time I've typed v, an Alfred-like window pops up that autocompletes the full folder names as I type - so by the time I get to 'fil', it's autocompleted to the right one and I can hit enter. Standard keyboard shortcuts too like j to junk, a to archive, etc. I'm sure Thunderbird also has these via extensions, but I like having it in the core product supported by the core developers.

There's a word count warning feature that I use a lot too, because I tend to type a lot. The word count turns red when I go over my suggested self-imposed limit, or you can set a timer for when you've been composing one email for too long.

EDIT: I saw you asked elsewhere about editing the From field, Postbox has a pulldown menu on each message you compose where you can choose from a list of profiles / aliases you create. Each can even have their own SMTP server details & default signatures & are separate from inbox accounts. You can switch signatures on individual messages too.

[1] https://www.postbox-inc.com/features

[2] https://www.alfredapp.com/

Thanks. In fact the time-limits on writing an email is a great idea! I should generalize that into a timer app for writing email, browsing HN, Wikipedia, etc.

Alfred looks interesting. Other than the workflows, almost everything it does is already integrated into KDE. However it looks easy to use and I will definitely check it out if I move to one of the company Macbooks. Thank you!