Office 365 offers multiple options for providing an encrypted mail service to users. Office 365 Message Encryption (OME) offers the best flexibility as Office 365 automatically trusts many of the main stream mail system providers for message decryption. S/MIME still requires a complex certificate exchange to be done between sender and recipient to ensure messages can be decrypted.
Mail providers Outlook.com, Yahoo and Gmail are automatically trusted by Office 365 and can automatically decrypt messages.
Other mail services which cannot automatically decrypt OME messages present a different set of challenges. One Time Passwords (OTP) are issued by Microsoft when an encrypted message is received by a mail server that cannot automatically decrypt messages. The OTP is sent directly from Microsoft to a user’s inbox (using the email address which was set as the recipient of the encrypted message). This way Microsoft can authenticate that only the intended recipient has received the OTP to decrypt the secured message.
Office 365 OME is a feature which is available in the following SKU’s Office 365 E3/E5, Microsoft E3/E5, A1, A3, A5 and G3/5. No licenses are required on the receiving side to accept encrypted mail. In this example we are going to make use of the Microsoft managed keys, however understand that it’s possible to BYOK to Azure Information Protection.
Go to the Azure Portal when you are logged in with your Office 365 credentials. Find Azure Information Protection from the portal.
Click on Protection Activation, if this is not active, go ahead and activate it.
Open the Exchange Admin Center and go to Rules. We are going to create a new Transport Rule to apply the encryption policy to emails that contain certain words. Click the + icon and select Apply Office 365 Message Encryption and Rights Protection to Messages.
Label the Transport Rule something sensible and click the Apply this rule if...and select The subject or body...then subject includes any of these words.
Enter all the key words in which you want to be used to encrypt mail. I think it's generally good to use "encrypted", if you do this ensure you put in all the obvious spelling mistakes for "encrypted" so that users don't send clear-text messages because of typo's.
It's very easy to send encrypted email from Outlook with a Transport Rule in place to apply the encryption rule. Simply ensure the work "encrypted" is in the subject line, this words that is looks for before encrypting the message are user-defined in the steps above.
When a recipient is also on Office 365 the message is automatically decrypted. You will notice the red marker to outline that this message is encrypted. Please note that when a single message (or reply) is encrypted, the entire conversation/message chain is encrypted.
If an encrypted message is viewable to a recipient, it's automatically opened into OWA. For some reason the message does not open directly in Outlook. This does ensure that the encrypted message cannot be forwarded.
What happens if your send an encrypted email to a mail service which cannot automatically decrypted the message?
The encrypted message is opened in the browser.
The OTP is sent to the inbox.
The recipient can respond to the message securely in the browser.