It‘s coming along in Thunderbird, they continuously mention it in their monthly development blog.

Exchange Web Services support in Rust

November saw an increase in the number of team members contributing to the project and to the number of features shipped! Users on our Daily release channel can help to test newly-released features such as copy and move messages from EWS to another protocol, marking a message as read/unread, and local storage functionality. Keep track of feature delivery here.

If you aren’t already using Daily or Beta, please consider downloading to get early access to new features and fixes, and to help us uncover issues early.