When you purchase this option, we force incoming and outgoing mail servers to use encryption to transport your messages. Unencrypted transport of e-mail messages in plain text will then not be allowed.

This can be used to create a special protection of messages, for example in sensitive industries like finance or health, etc.

Explanation:
While Mailix pays a lot of attention to encryption and security, for historical reasons there are still some mail servers on the Internet that do not understand encryption and can only transmit and receive messages in plain text. Therefore, in order for us to transmit messages in a generally reliable and trustworthy manner, we unfortunately have to allow unencrypted transmission to such mail servers.

If you select this option, we will not allow unencrypted transmission of your messages and will deny transprt to mail servers that do not support encryption. The disadvantage is that you cannot send messages to users of such mail servers and you cannot receive messages from them. On the other hand, all your messages from and to you are guaranteed to be encrypted during the whole transport (and on our servers anyway).

See also:
Mailix Documentation: Encryption