One of my contacts recently changed emails. Updated contact info in Contacts and then tried to send a message to his @comcast.net email address (he does not have an iPhone). I keep getting a "XXX_XXXX@comcast.net is not registered with iMessage error" when trying to send the message. Strange thing is, there is no problem sending the message to the same address when using my iPhone. Tried the usual: deleted iMessage account and re-set it. Restarted computer. Installed latest security updates. Logged into same iCloud account on my iPhone and Mac. Nothing works. Emails still go out to everyone else just fine, regardless of whether a phone number or email address is used.