I sign my emails with S/MIME (don't usually use it for encryption) and have enabled S/MIME in Zoho both centrally and for me as a user. Up until yesterday, it was working fine, but when I tested today I keep getting message in Apple Mail that the content cannot be verified,  even though the S/MIME icon is there and ticked as though everything is fine and the email is signed. 

It says "Unable to verify message content" and there is a button also: "Load Message (Not Recommended).  When I load the message, it opens without issue and the unable to verify message goes away. 

I don't know how to even begin to troubleshoot -- please help. 

Thank you