An important security feature that O365 has is the ability to encrypt emails.

This is important when sending sensitive information such as banking/payment details, confidential data, etc. to anyone but especially to outside of our company. 

Note:  You have to be assigned an O365 E3 license in order to have this feature.  If you don't see the Encrypt option in this area, more than likely you are assigned an O365 E1 license.  If you job deals with sending sensitive information then please put in a help desk ticket asking to be upgraded to an E3 license.  This will have to be approved by a manager.  

To encrypt an email you click on Options when creating the email – pick Encrypt and then Encrypt-Only.  The “Do Not Forward” also encrypts and keeps anyone from forwarding the email.