I have (kind of by accident) found someone who made a post about making Mailspring work with institutional Office365 using the Exchange Web Services, which I guess a lot of people in here could take advantage of until EWS is implemented in Mailspring. It makes use of DavMail Gateway, a cross platform app that creates a local POP/IMAP/SMTP/etc email server connected to the EWS Microsoft account.
I just downloaded it for Linux, using Mailspring 1.10.0 from Github, and it works like a charm. You just have to make sure the ports match, that you select no SSL, and use ‘localhost’ as server.
Of course, I can’t guarantee that nothing would be problematic with this workaround, as I don’t know anything about email protocols. If anyone knows about possible problems with this implementation, please comment!