Encryption with RMail is easy, just check off the box next to the word "Encrypt" in the feature panel pop up window after you hit the "Send Registered" button.

There are, however, a few options to consider when choosing the Encrypt feature
If the sender chooses to use the Encrypt feature for their email, RMail will by default send the message using the secure transmission protocol Transport Layer Security (TLS), which does not require a password for the recipient to open the encrypted message. Both the encrypting and decrypting are done at the server level and nothing is required of the receipt.
However, in the case where the recipient's email system is not using TLS, the email will go by Message Layer Encryption and the recipient will be required to enter a password to open the email.
In this latter scenario, (when the recipient does not have TLS and a password is required to open the message) RMail offers the sender two methods for creating and delivering passwords to the recipient, the first option is Automatic, whereby the sender does nothing and the system manages the entire process. The second option is called Custom, whereby the sender must pick a custom password for the recipient to use to open the message. The differences are explained below.
Automatic
RMail provides the recipient with a random system-generated password which is automatically delivered by email to the recipient. The recipient uses the password to open the email.
Custom
The email is sent by TLS but the sender can choose a custom decryption password for the recipient
NOTE! Email custom password box must be selected if you wish to have this password sent to the recipient automatically -if it is not checked, the sender must provide the password to the recipient by other means.
For additional information on the types of encryption and their options and settings, please reference the downloadable guide attached to this article.
RMail - A deeper look at Encryption Options