The Below information is from Zix Directly:
Monitoring - Our Engineers are still looking into the cause of the issue, but we are getting reports that the rejections have subsided.
Dec 16, 15:22 CST
Investigating - We are aware of an issue with outbound messages being rejected. We have raised a high severity ticket with our engineers and they are currently looking into the issue.
Dec 16, 15:09 CST