Email protocols: Microsoft supported POP3, IMAP, and SMTP email protocols in their Microsoft Outlook email client. At the same time, they developed their own email protocol, MAPI, which has since been documented but is largely unused by third parties. Microsoft has announced that they would end support for basic authentication access to Exchange Online APIs for Office 365 customers, which disables most use of IMAP or POP3 and requires significant upgrades to applications in order to continue to use those protocols;[23] some customers have responded by simply shutting off older protocols.
I was required to implement this (IMAP with OAuth2) "simple change" but for a server backend service that checked an inbox to perform certain actions. That was certainly fun. In the end the solution wasn't that difficult, but finding it and working it into legacy code...