We are not sure the cause as of this time, but it looks like exim (the software that operates the email server) needed to be reinstalled and restarted. Once we did that it was back up and running. All email that was sent while it was down was bounced back to the recipient with a 550 code (which oddly doesn’t exist in exim).
This means that those emails will need to be resent. Emails that you sent were queued and have been sent out. This is a strange one and we have opened a ticket with exim support to help us investigate.