Opportunistic TLS
With opportunistic TLS, if a connection attempt is made using the TLS protocol, the connection recipient must provide appropriate TLS credentials for an encrypted data transfer. If the TLS “handshake” fails, the data transfer is made via plain text, rather than encrypted text. In either case, the data transfer is successfully accomplished.
Opportunistic TLS is used for message routing transfers. Use the page to create a new route that uses the TLS delivery option or edit an existing mail route to add the TLS option. At the bottom of the page, mark the check box Use Transport Layer Security (TLS) to use opportunistic TLS for message routing.
The third-party application message encryption feature also uses opportunistic TLS for data transfer security. Third-party application encryption options are configured on the page .