For a complete reference on Office 365 Message Encryption technology, please visit this URL https://products.office.com/en-us/exchange/office-365-message-encryption
Steps for sending an encrypted email in Outlook:
Upon drafting a new email message, click on the "File" tab.
Next, click on "Properties.
Under the message settings, change the sensitivity level, to "Confidential".
You can now click close to return to the email message. Any content, including attachments will be sent encrypted.
Steps for sending an encrypted email in OWA:
Start a new email and click on the "ellipses" for more message options.
Now click on "Show message options..."
Under the message settings, change the sensitivity level, to "Confidential".
You can close this dialog window now and send the email encrypted.