If you missed it, the issue was caused by a date check failure in Microsoft's Exchange server antivirus engine with the switch over to the year 2022. For the solution, Microsoft says IT admins can now download an automated script.
To get the fix, IT admins can follow these steps. First, change the execution policy for PowerShell scripts by running Set-ExecutionPolicy -ExecutionPolicy RemoteSigned. Next, the script can be downloaded with a simple click. Finally, the script can be run on each Exchange mailbox server that downloads antimalware updates.
There is also a manual solution available, should an admin prefer. According to Microsoft, the solution for this problem is currently automated because it might take some time to make changes, download updated files, and clear transport queues. The company, though, makes it clear that any manual or automated solution must be performed on every on-premises Exchange 2016 and Exchange 2019 server. The script can even be run simultaneously on multiple servers.
In all cases, using Microsoft's emergency script might take some time to run. The script might also take some time to clean up any missed messages in the transport queue. Reaction to the script/fix has been mixed, with some saying it works, and some saying it does not. Others report that running the script multiple times could help fix the issue.