Looks like Apple’s iMessage is down for the count, as noted by numerous users. Messages look like they are about to finish sending, but they don’t completely go through. Our own experience trying to send iMessages on our Mac gives us a message that particular users “aren’t registered with iMessage” (which isn’t true):
The last time an iMessage outage hit was back in mid-September. During times like these we usually fall back to WhatsApp or even Skype. Is iMessage down for you?
Update: The outage appears to be over for the majority of users.