Unfortunately I didn't find out til after I'd spent a good hour logging into lots of websites and updating my email address to my shiny new Protonmail ones, that I can't connect to a free tier Protonmail account with IMAP and Thunderbird. D'oh!

Protonmail looks like a good choice, if you only have one or two email addresses and don't mind being restricted to accessing it via webmail. But I've got half a dozen or more across various services, as well as on my own domains and I want to continue to be able to access them all from within one app [ie. Thunderbird]. The whole different app for every web service thing gets tired very quickly. Also, as per the original thread starter, if you only access your Protonmail account via webmail this does still leave you at the mercy of another company which may or may not decide to lock you out.

Can anyone recommend any free alternatives to Gmail / Yandex Mail [preferably privacy respecting] that will actually integrate with an IMAP client?