Virtru has a nice balance of plug-ins and mobile apps that support its easy-to-use encryption operations across a variety of email circumstances. If you have installed the necessary plug-in, when you want to send something, there is a small toggle switch on the top of the compose screen. Turning that on will bring up a “send secure” button to encrypt your message. There are tool tips that appear as you hover over the various options with your mouse, a nice touch. These include the ability to add an unencrypted introductory message that will introduce your recipient to the context of the message that you are sending, and why you want to encrypt the remainder of the message. You can also set when your message will expire or disable any forwarding for additional security.
Virtru also supports zero knowledge encryption, although it adds a separate activation step when a new user receives the first encrypted message.