Exchange Internal Mail - 400 4.4.7 Message Delayed

In my Exchange 2016 environment I have three multirole servers in a DAG, running on VMWare. Everything had been humming along nicely until everyone started getting the following bounce message:

4/17/2017 9:09:53 PM - Server at MBX1 (10.28.69.162) returned '451 4.4.0 Target host responded with: 4.4.7 Message delayed'

This was for internal messages, between the three Exchange servers. Which is pretty odd, since they're set up to transfer mail out of the box.

I also found the following Events on the servers:

Log Name:      Application
Source:        MSExchangeTransport
Date:          4/17/2017 5:20:52 PM
Event ID:      1035
Task Category: SmtpReceive
Level:         Warning
Keywords:      Classic
User:          N/A
Computer:      MBX1
Description:
Inbound authentication failed with error UnexpectedExchangeAuthBlobCheckForClockSkew for Receive connector Default MBX1. The authentication mechanism is ExchangeAuth. The source IP address of the client who tried to authenticate to Microsoft Exchange is [10.28.69.163].

As you can see, the "ClockSkew" part clues us in what's going on.
The problem was that the Time sync on the VMware hosts wasn't set up correctly, so the system clocks on the Exchange guests were off...way off.

Anything over 5 minutes will generally cause issues, with everything from mail routing to DAG replication.

To fix it, follow my previous post here on fixing the VMWare host time issues.