.Net Framework 4.6.2 breaks Exchange 2016

We ran into a tricky problem with a .Net 4.6.2 upgrade and Exchange 2016.  The April 2017 Windows updates include an upgrade to .Net 4.6.2.  Unfortunately this will break Exchange 2016 unless you have at least Exchange 2016 Cumulative Update (CU) 3 installed in the Exchange 2016 server before you install .Net 4.6.2.  This particular Exchange 2016 Server only had CU1 installed on it.  If you install .Net 4.6.2 without Exchange 2016 CU3 or later, the Exchange server will become unresponsive, mail flow will stop and many of the Exchange Services may fail to start up. 

The server was so unresponsive we couldn't even uninstall .Net 4.6.2. Fortunately this Exchange Server had the OS installed on C: and the Exchange databases installed on D:.  This Exchange server is virtualized so we were able to easily restore just the C: drive using Veeam.  Here are the steps we took to fix the problem:

  1. Restore c: drive of the Exchange Server from the previous night's backup.  This left the Exchange databases in place so we didn't lose any mail.
  2. Install Exchange 2016 CU4.  It's recommended to upgrade to Exchange CU4 before upgrading to .Net 4.6.2
  3. Install .Net 4.6.2.  Because CU4 is compatible with .Net 4.6.2 we could now upgrade .Net.
  4. Install Exchange 2016 CU5.  This is the latest Cumulative Upgrade for Exchange 2016 as of today.

If anybody else runs into this issue, hopefully this Blog will help.  The solution isn't too difficult after we figured out the root cause of the problem.


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