I'm fed-up with Thunderbird and am switching to mutt. Not sure whether this will be permanent or perhaps a step towards nmh, the new mail handler. What I know is Thunderbird is slow to start-up and while vastly superior to Office 365 (which we've begun using at work), it's still no match for a good command-line UI.
I've also been experimenting with nmh with mairix. To get there I need to convert my mail from Maildir to mh. It's actually pretty easy to do but I want a utility that will do the work for me. I'm looking at creating something in Python. Python has an excellent library for handling mailstores.
Regardless of which mailstore I choose, Mutt will work with it so for now, mutt is my mail client.