Note - the feature called "disable forwarding" in Virtru does not truly prevent someone from forwarding the email. Instead, the email can still be forwarded, but additional recipients will be unable to decrypt it.

1. On the top right of your email compose window, toggle the Virtru slider ON. 
2. Click the arrow to the left of the slider.
3. Check the round checkbox to the right of Disable Forwarding.
   

More Information

 For more detailed instructions, see Virtru’s article How Does Disable Forwarding Work?