Jason C. answered 08/18/20
Microsoft Certified Outlook Expert with University teaching experience
This may sound basic, but have you checked the syntax of your email recipient's address? Another issue may be that there is either a space or hidden character before or after the email address on your contact card. A third scenario may be that your contact card is corrupt and needs to be rebuilt.

Alan E.
Thanks very much. I'm sure you are right about the cause. I will fix it. Best wishes. > Alan08/18/20