Do you have the unresponsive owner's written permission to receive email communications from the road association. If you do, I would keep on record hard copies of the email notifications, results of voting, etc., you send.
If you have not received written permission then, yes, send notifications by certified U S mail. We also have one owner who requires U S mail communication. Certification is not required but being sent to the current address listed at the Town assessor's office is required.