Troubleshooting - Mail Client Connectivity and Mail Flow

We have an On-Premise Exchange Server 2013.  Over the last few days, the mail clients aren't connected to the Exchange database and mail flow has stopped.  

[PS] C:\Windows\system32>Test-mailflow
RunspaceId         : ecfa2d61-ec65-41ae-bc63-2c1d6261b153
TestMailflowResult : *FAILURE*
MessageLatencyTime : 00:00:00
IsRemoteTest       : False
Identity           :
IsValid            : True
ObjectState        : New

Also, Delivery Reports are throwing the following errors:

Server xxxxx.domain.com in domaindomain.com encountered an unexpected error. Diagnostic information for this error: The message-tracking data on serverxxxxx.domain.com had invalid data in column 12 for this message. Event "THROTTLE" will be ignored.

Any suggestions?

Thanks.