Met with various email providers last year in London among them the FastMail team the ones pushing it, eM Client and some other major cloud/hardware providers. We talked about JMAP and importantly "dynamic oauth for clients" which will allow any provider to launch their own login pages within clients much like what gmail and outlook do now. We don't have any support yet for jmap but it does seem exciting we just have our focus on other parts of the application atm. We've done some experimental testing on the new oauth mechanism.