In case you run Exchange on-prem yet. Microsoft has a bit of a date problem that prevents mail delivery. To get messages moving again you need to bypass their malware filtering. Happy New Year from Microsoft 
Set-MalwareFilteringServer -Identity -BypassFiltering $true
Restart-Service MSExchangeTransport
https://www.reddit.com/r/sysadmin/comments/rt91z6/exchange_2019_antimalware_bad_update/
@chaddb Thanks for the heads up!! Hopefully you (none of us) got caught up in this!
Currently we seem to be working just fine. I’ll need to check on that patch and see if it’s installed or not.
We run on-prem Exchange yet and were caught up in this mess. The Reddit article and BleepingComputer article hadn’t been published yet when I started looking into why our email had stopped moving. So I didn’t have much to go on other than Exchange logs. I ended up adding the bypass that Microsoft ended up posting as the workaround before they did lol.