This error message has started appearing since Oct 2 on valid email addresses in existing customer contacts that have been emailed from Zoho CRM weekly for over a year.  Each week Zoho reports this error with more email addresses. These email addresses can be successfully emailed from other email providers.  What's up with Zoho CRM?  How does Zoho determine this?  What checks is it performing?  Are there any valid reasons for this error to appear on a valid email address?

This is rapidly becoming a very serious problem.  This error message was never seen prior to October 2015 during extensive Zoho CRM use for many years.  Did Zoho CRM software change?

Thank you for any valid insights.