Microsoft has made some updates to the way Outlook communicates (MAPI-over-HTTP) with its popular Cloud Exchange service Office365 and as of October 31, 2017, they will stop allowing the old method (RPC over HTTP) to connect at all.
There are patches available for Outlook 2010 and up that must be applied before the Oct 31st deadline. Users of Outlook 2007 and below are going to be forced to upgrade from their old outlook versions to a newer version of Outlook if they plan to continue working with Office365 for mail.
