552 5.2.0 bounce error sending group emails!

This is interesting info.  My problem started on or about 25 January.  Using my Outlook 2016 program I sent out an email with a distribution list of 11 people.  Instantly it bounced back ("System Administrator-Undeliverable"), code 552 5.2.0, internal error.  I called COX and explained the situation.  They said it was an Outlook problem and I needed to call Microsoft.  I was never told there was now a limit on the number of recipients per email message. After reading this thread I now I know why.  My guess is this is a financial decision on their part to try to limit email traffic thereby controlling the number of servers and associated hardware they need to keep the system up and running.  Unfortunately for the most part I've lost the functionality that Outlook provides and forced to use COX webmail and its shortcomings for emails with distribution lists exceeding 9.  Thanks COX!!