Mail Merge

I was using Office 2016 until about mid October when my mail merges from Word using an Excel spreadsheet as data source for the email addresses failed to work. On Microsoft advice we recently upgraded to Office 365.

Microsoft support has not been able to diagnose the problem and has suggested I ask the community. A case summary is attached. 

Help would be very much appreciated as I usually use mail merges a lot to contact members about subscriptions etc and to send emails about event bookings.