Office 365 Outlook 2007 Compatibility
If your customers have been hedging on Office versions, here's the answer to the question, "How long can we leverage our Office 2007 investment?" A: 31-Oct-2017.
That's not the end of the story, though. You'll need to pay attention to your version numbers for Office 2010/2013/2016, also. The table of minimum versions, along with the good reasons to get off of the old RPC-over-HTTP method in favor of MAPI-over-HTTP, can be found in the official announcement.