As of Fall 2019, to send a secure email with Microsoft Office Message Encryption, you will simply need to put the word SECURE (case-insensitive) in the subject line of your email.  

Additionally, there is the option to set the permission level on the e-mail, which will force Encryption to external receipients. The first time the Permission button is used, you will need to click to connect to the Rights Management servers.

Unrestricted Access does NOT encrypt a given e-mail, and therefore should NOT be selected when sending Encrypted e-mail. Encrypt-Only has a less restrictive permission set than Do Not Forward, as Do Not Forward will prevent downloading of the message as well as forwarding to additional recipients.