Lumia 950 :: Emails Not Sending Error (Gmail / Exchange)

Got a new Lumia 950 about 10 days ago. Set up my 2 email accounts (Gmail, Exchange) and, initially, they both worked correctly. I was receiving messages and could reply.

Starting two days ago, out of the blue, I can no longer send messages. I am still receiving messages on both accounts. But, attempts to reply (or send a new message) result in a red "x" showing up in my inbox (next to my sent message). When I click on the message that did not send, it says "There was an error sending your message."

I just tried deleting both email accounts and reconnecting them. No luck.