On October 31 2017, RPC over HTTP will be deprecated in Office 365, as stated here. This means that older Outlook versions (those that use RPC over HTTP) will no longer be able to connect to Office 365. The reason for this is that RPC over HTTP will be replaced in favor of the much newer MAPI over HTTP.

This switchover has an impact on all (older) Outlook versions that do not have the required update (incorporating MAPI over HTTP) functionality installed. Only Outlook versions that get this update will be able to connect to Office 365, resulting in the following changes:

• Outlook 2007: no longer supported (there is no update).
• Outlook 2010: only supported with SP2 and KB 2878264 installed (additional fixes in the KB 2956191 update).
• Outlook 2013: SP1 required.
• Outlook 2016: supported.

Although the article from Microsoft states all Outlook versions must have the "December 8, 2015 update" installed, there's no mention of this in the detailed article here.

Note that on-premise Exchange currently is not affected: RPC over HTTP will continue to function.