As many of you should know the .NET Framework 4.6.1 has been sent to Windows Update as a recommended update. However there are some issues with Exchange, such as Mailboxes getting quarantined and databases failing over unexpectedly in Exchange Server 2013, described in https://support.microsoft.com/en-us/kb/3095369 and so we ask you to refrain from updating for the time being.
I urge you to read all the details at :
On .NET Framework 4.6.1 and Exchange compatibility – Exchange Team Blog – Site Home – TechNet Blogs
For now you can prevent the deploy of the mentioned update by applying the steps described in https://support.microsoft.com/en-us/kb/3133990 .
Have a great day.