Transport Layer Security (TLS) is a successor to Secure Sockets Layer protocol (SSL). TLS provides secure communications over the Internet for such things as e-mail and other data transfers.  It enables privacy, integrity and protection for the data that's transmitted between different nodes, or mail servers over the Internet. 

The TLS Handshake Protocol enables the client and server to authenticate each other and select an encryption algorithm prior to sending the data.  When a server and client communicate, TLS protocol ensures that no third party may eavesdrop, tamper with any message, and message forgery.

RMail sends all messages by TLS by default.  If the recipient is not able to receive emails by TLS, RMail delivers the message over a non-secure connection except with the RMail Encryption service where a 256-bit AES encrypted PDF wrapper protects the sender’s email and attachments.