**UPDATE** October 12, 2017
Microsoft has now released an update to their earlier announcement. They have clarified that, as of October 31, RPC over HTTP will no longer be supported on Office 365, but the protocol will not actually be removed. This means that Outlook 2007 will not suddenly stop working as a mail client against Office 365 as of October 31. It does mean that "the quality of the mail experience will decrease over time," as it will no longer be a supported protocol, and Microsoft will no longer be issuing code fixes or updates for non-security-related problems.
In early July, Microsoft made an announcement that you may not have noticed. As of October 31, 2017, RPC over HTTP – which several generations of Outlook have used to connect to Exchange servers remotely – will no longer be supported in Office 365 (or Exchange Online, if you’ve subscribed to that as a separate service). Instead, Outlook clients will have to use MAPI over HTTP, which was introduced in May, 2014. This has important implications for all subscribers (and prospective subscribers) to these services. Read More