Thanks, keeping this as a reference — I'm trying to find some time to try mu4e. I used Gnus for many years, then switched to Apple Mail.app, but with the gradual decline of MacOS (and Mail.app) I'm looking to switch back.

I remember the two main reasons I switched from Gnus: 1) there was no good reliable search, 2) I couldn't drag&drop attachments into E-mails and back so I was spending a lot of time pointing to files. I hope both things have improved since then.

Have you tried using the hardcoded Thunderbird (or similar) oauth credentials to authenticate to Google et al? You can also use davmail to proxy your requests to Office 365 / Exchange and it handles oauth also.

For OAUTH2, I recommend starting pizauth (https://github.com/ltratt/pizauth) as a user systemd unit.

Then you can just do (eg. in mbsync)

PassCmd "pizauth show accountname"