What makes you say the protocol is different for each provider?
I maintain a proxy that transparently adds support for OAuth 2.0 support to IMAP/POP/SMTP clients (https://github.com/simonrob/email-oauth2-proxy), and for normal use it doesn’t need to know anything about which service it is connecting to. Apart from advanced features such as CCG or ROPCG which are mostly O365 only, what is different?
I maintain a proxy that transparently adds support for OAuth 2.0 support to IMAP/POP/SMTP clients (https://github.com/simonrob/email-oauth2-proxy), and for normal use it doesn’t need to know anything about which service it is connecting to. Apart from advanced features such as CCG or ROPCG which are mostly O365 only, what is different?