> FIDO U2F support Sweet, although IIRC several of the email providers with 2FA (Gmail and Outlook come to mind) have the option of providing app-passwords instead, which bypass the need for a 2FA token.
Just so you know, an app password downgrades the security of your 2FA+Password pairing, and I never use app paswords because of that. If it was somehow possible to intercept the password used in the IMAP handshake, then that means access to your inbox without 2FA. This is why I am a huge fan of web-based clients and not things like Thunderbird.
A false dichotomy. 2FA login is achieved through "web login", where an authentication token is obtained by presenting the service's 2FA interface in a web frame or a simple web browser. See e.g. the initial setup flow in Android 4.x - the Google login and password are a regular form, but if 2FA is enabled, you're shown a web browser.
Whether clients like Thunderbird implement web login remains a quality of implementation issue...