Why Some Outlook Clients Encrypt Outbound Messages Differently

Outlook Sensitivity Labels Processed in Different Ways

An observant reader noticed that Outlook clients encrypt messages using sensitivity labels in different ways. If you look at Figure 1, you see three messages sent to the same person using Outlook Mobile, OWA (or Monarch), and Outlook for Windows. The Ultra Confidential sensitivity label protects all messages with encryption, but only the copy sent from Outlook for Windows is protected in the sender’s mailbox. The other copies sent from Outlook Mobile and OWA are protected when they arrive in the recipient mailbox.

Outlook lists three messages from different clients with different outcomes from Outlook sensitivity labels
Figure 1: Outlook lists three messages from different clients

The obvious question is why this situation happens. Shouldn’t all Outlook clients produce the same result? Alas, this is not the case. As explained in Microsoft documentation, “When a sensitivity label is configured with encryption, the encryption process depends on the client platform.” In effect, Outlook desktop is the only client that contains the code necessary to encrypt an outbound message.

Other Outlook clients rely on passing messages through the Exchange Online transport service. The transport service has super-user capabilities and can apply the necessary protection. When transport detects that a message has a sensitivity label with encryption that isn’t yet protected, it does the necessary work to protect the message by placing the message and its attachments in a rpmsg “wrapper” before sending the message on to the next hop in its journey.

Client Processing for Protected Messages

The rpmsg wrapper is how Outlook sensitivity labels impose rights management for protected messages. The receiving client must unpack the message from the wrapper and respect the rights assigned to the recipient by the publishing license that’s included in the wrapper. The receiving client sends the publishing license to the information protection service to obtain a use license that allows the client to open the message.

Clients perform the processing to allow users to read protected messages without being prompted for credentials. If the client can’t obtain a use license, it displays information from the rpmsg to direct the user to the Office 365 Message Encryption (OME) Portal. If the user can prove their rights to open the message by signing into the OME portal with an account included in the recipient list, they can view the message contents online.

The reason why two out of the three messages are unencrypted in the Sent Items folder is that these are the messages that clients didn’t protect. Outlook desktop protected the other message before it submitted the item to transport. In

all cases, the sender can be confident that the message was fully protected when it left the transport service for onward routing.

Clients and the MIP SDK

Microsoft could incorporate the code (using the Microsoft Information Protection SDK) to protect messages in OWA and Outlook mobile. However, this approach doesn’t seem to make sense. Apart from the extra complexity introduced into the client code base, OWA can only be used online. Outlook mobile clients could protect files, but they usually work in a connected mode (either Wi-Fi or a cellular network). Outlook desktop has always been able to work offline, so its developers incorporated the code to process protected inbound and outbound messages when working offline.

Growing Use of Outlook Sensitivity Labels

The number of messages protected by Outlook sensitivity labels is steadily increasing. I do not have firm data to back this assertion, just anecdotal evidence from customer interactions. Microsoft continues to pour engineering effort into making sensitivity labels more accessible and useful, so I expect the trend to continue. And when your tenant starts to use sensitivity labels to protect email, you’ll know why some Outlook clients protect messages in a different manner to others.

Learn about using Exchange Online, Outlook clients, and the rest of Office 365 by subscribing to the Office 365 for IT Pros eBook. Use our experience to understand what’s important and how best to protect your tenant.

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