Symptoms
Simple mail transfer protocol (SMTP) uses transport layer security (TLS) 1.0 in a Microsoft Exchange Server 2013 environment, even if you have enabled TLS 1.1 or TLS 1.2.
Cause
This issue occurs because of a hard-coded restriction that limits SMTP to use secure sockets layer (SSL) 3.0 and TLS 1.0 for transport.
Resolution
To resolve this issue, install the following cumulative update:
Cumulative Update 8 for Exchange Server 2013Note This update removes the hard-coded restriction so that SMTP sessions can negotiate with any SSL or TLS protocols that are enabled in the host operating system.
Status
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
More Information
For more information about TLS and SSL protocols, see What is TLS/SSL.For more information about how to enable TLS 1.2 protocol in registry key, see TLS/SSL Settings.
