Transport Layer Security TLS is a standard protocol that’s used to provide secure Web communications on the Internet or intranets. It enables clients to authenticate servers or, optionally, servers to authenticate clients. It also provides a secure channel by encrypting communications. TLS is the latest version of the Secure Sockets Layer (SSL) protocol.
How Does IT Work?
You need to have a certificate, it can be self-signed or 3rd Party purchased.
You have to make sure the certificate is assigned to the SMTP service like other web services.
Once you are almost ready and now you have to test it. The best way to test is using Telnet on port 25 of the receiving server and it should return 250 STARTTLS as below
Now let us check the setting at tthe Exchange Server, in this case I am using exchange 2010. Keep in mind Exchange 2010 uses Opportunistic TLS, mean by default it will try to establish the connection on SMTP on TLS and if not then it will regular SMTP.
Example of non-TLS non -secured SMTP message between two Exchange Server 2010
Example of secured message SMTP over TLS
Simple Right 