I have a contact who has a cell phone number and a mac.com email address. When I use the Messages app to send him messages, he doesn't always receive the messages. (I've been using the app on macOS and iOS.) I believe the successfully messages are sent to the phone number while the failed ones are sent to the mac.com address. I can see this in the desktop app where there is a header above groups of messages saying "SMS with email@example.com" or "SMS with +1 (555) 555-5555".
Is there a way to tell the Messages app to only use the phone number, and ignore the mac.com email address?