Microsoft has released .Net Framework 4.6.1 and this doesn’t go well with any version of Exchange server, including 2016 yet. You need to watch out for this one especially if you use Windows update to patch the servers.
The 4.6.1 update to the .Net Framework comes as a recommended update in Windows update and hence you need to uncheck the box before patching the server. The same goes for any update management solutions like WSUS or SCCM. Customers are reporting that mailboxes are being quarantined and databases are getting dismounted & failed over unexpectedly after installing the update on Exchange 2013.
The resolution is to uninstall .Net Framework from the server, which will roll back the machine to the previously installed version (which should be 4.5.2).
[clickToTweet tweet=”No version of #MSExchange that has been released so far supports .Net Framework 4.6.1″ quote=”No version of Exchange that has been released so far supports .Net Framework 4.6.1″]
Un-installing gets a bit tricky depending upon the version of the operating system on which Exchange is installed.
- In Windows Server 2008 SP2 & 2008 R2 SP1, Microsoft .NET Framework 4.6 is installed as a product under Programs and Features in the control panel and hence it is easy enough to uninstall.
- In Windows Server 2012, Update for Microsoft Windows (KB3045562) needs to be removed from the Installed Updates in control panel.
- In Windows Server 2012 R2, Update for Microsoft Windows (KB3045563) needs to be removed from the Installed Updates in control panel.
In medium to large organizations, patching of servers is managed by a different team. As an Exchange admin, you can protect your servers from .Net 4.6.1 installation (that might happen without notifying you as it is released as a recommended update) with a registry edit.
- Start Registry Editor.
- Locate and click the following subkey
- HKEY_LOCAL_MACHINE\Software\Microsoft\NET Framework Setup\NDP
- Select the subkey & point to New on the Edit menu, and then click Key.
- Type WU, and then press Enter.
- Right-click WU, point to New, and then click DWORD Value.
- Type BlockNetFramework461, and then press Enter.
- Right-click BlockNetFramework461, and then click Modify.
- In the Value data box, type 1, and then click OK.
- On the File menu, click Exit to exit Registry Editor.
It is not a nice place to be in, but it is better to be safe than sorry 
