New Outlook Mandatory Labeling: Stop sends until a label is applied

You have configured sensitivity labels in Microsoft 365 but users still send emails without applying a label. The new Outlook for Windows and Outlook on the web enforce mandatory labeling only when the Microsoft Purview compliance portal policy is set to block unlabeled outbound emails. This article explains how to configure the exact policy rule that stops email sends until a user selects a sensitivity label. You will learn the required license, the policy location, and the behavior users see when they try to send an unlabeled message.

How Mandatory Labeling Works in New Outlook

Mandatory labeling is not a setting inside Outlook itself. The new Outlook for Windows and Outlook on the web rely on Microsoft Purview Data Loss Prevention policies to enforce label requirements. When you create a DLP rule that blocks unlabeled outbound emails, the client intercepts the send action before the message leaves the outbox.

The enforcement only applies to messages sent from new Outlook and Outlook on the web. Classic Outlook for Windows does not block sends based on this DLP rule. Users of classic Outlook must rely on client-side add-ins or manual label enforcement.

The feature requires one of the following licenses:

• Microsoft 365 E5
• Microsoft 365 E5 Compliance
• Microsoft 365 E5 Information Protection and Governance
• Office 365 E5

Users must also have sensitivity labels published to them through the Microsoft Purview compliance portal. If no labels are published, the DLP rule cannot enforce labeling because there is nothing to select.

Steps to Configure Mandatory Labeling in Microsoft Purview

1. Sign in to Microsoft Purview
   Go to https://compliance.microsoft.com and sign in with an account that has the Information Protection Admin role or Compliance Admin role.
2. Navigate to Data Loss Prevention
   In the left navigation, select Data Loss Prevention. Then select Policies.
3. Create a new DLP policy
   Click Create Policy. Choose Custom as the template and give the policy a name such as “Require label on outbound email.”
4. Select the location
   Under Locations, select Exchange email. Do not select SharePoint, OneDrive, or Teams unless you want label enforcement on those workloads as well.
5. Define the rule conditions
   Click Next until you reach the Rules page. Click Create Rule. Set the rule name to “Block unlabeled outbound email.” Under Conditions, select Content contains. Then choose Sensitivity labels. Set the condition to “None selected.” This means the rule triggers when no sensitivity label is present.
6. Set the action to block
   Under Actions, select Restrict access or encrypt the content in Microsoft 365 locations. Select Block people from sharing and receiving email. Check the box for “Require a specific label to be applied to the email before sending.” Then select the label you want to enforce. If you want users to choose any published label, select “Any label.”
7. Configure user notification
   Under User notifications, turn on Notify users in Office 365 with a policy tip. Check the box for Notify the user who sent the email. The default text tells the user to apply a label. You can customize this text.
8. Set the rule mode
   On the last page, set the policy to Turn it on immediately. Click Submit.

What Users See When Sending an Unlabeled Email

When a user composes a new email in new Outlook or Outlook on the web and clicks Send, the client checks the DLP policy. If the message has no sensitivity label, a pop-up appears with the following elements:

• A warning that the message cannot be sent without a sensitivity label.
• A link or button labeled Select sensitivity label.
• A Cancel button.

The user must click Select sensitivity label, which opens the sensitivity bar. After picking a label, the user clicks Send again. The DLP policy re-evaluates the message, and the send completes.

If the user clicks Cancel, the message remains in the draft folder unsent.

If Mandatory Labeling Does Not Block Sends

Users can send emails without a label

The DLP policy may be in test mode. In Microsoft Purview, open the policy and check the Mode setting. It must be set to Turn it on immediately. Test mode only logs the violation and does not block the send.

The DLP rule condition does not match

The condition must use “Content contains Sensitivity labels” set to “None selected.” If you set the condition to “All” or a specific label, the rule will not trigger for unlabeled messages. Verify the condition logic in the rule editor.

Users use classic Outlook for Windows

Classic Outlook does not enforce this DLP rule. Users must switch to new Outlook or Outlook on the web. Alternatively, deploy a client-side add-in such as the Microsoft Purview Message Encryption add-in to enforce labeling in classic Outlook.

No sensitivity labels are published

If no labels are published to the user, the DLP rule cannot enforce labeling. In Microsoft Purview, go to Information Protection > Labels, select a label, and publish it to the user or group. Then wait up to 24 hours for the labels to appear in Outlook.

New Outlook vs Classic Outlook: Mandatory Labeling Behavior

Item	New Outlook / Outlook on the web	Classic Outlook for Windows
DLP enforcement	Blocks send when DLP rule matches	Does not block send
User notification	Pop-up with Select sensitivity label	No pop-up
Label selection method	Sensitivity bar in compose window	Add-in or ribbon button
Policy configuration	Microsoft Purview DLP	Client-side add-in or registry

What to Do After the Policy Is Active

Test the policy by sending an unlabeled email from a test account in new Outlook or Outlook on the web. Verify that the pop-up appears and the send is blocked. If the policy works, notify your users about the new requirement. Users can apply a label before composing by clicking the sensitivity bar at the top of the message window. The label appears in the message header and in the recipient’s email client. For advanced scenarios, you can configure automatic labeling rules in Microsoft Purview to apply a default label when the user does not pick one.