I apologize this is happening. I just want to make sure, did you check the Junk/Spam folder of the e-mail address you were sending to? And do you know if the firewall security for your e-mail servers has been updated recently? It's possible it's getting blocked by your e-mail security.
If it's not there I recommend calling our Deliverability team at 1-866-433-8499 between 7AM and 9PM Monday through Friday. They will best be able to assist you.