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Custom Oversharing Dialog Works in the New Outlook Client
Given that message center notification MC1202974 (Microsoft 365 roadmap item 498921) appeared on 22 December 2025, I might have been forgiven for overlooking the announcement that the new Outlook for Windows client (aka Monarch) will support custom oversharing dialog for Data Loss Prevention (DLP) policies by the end of January 2026. But I have written about oversharing dialogs before, so it seemed like a good idea to examine how the custom variation works.
What an Oversharing Dialog Does
In a nutshell, an oversharing dialog is a pop-up invoked by an Outlook client when a DLP policy detects a potential rule violation in a message or its attachments. The idea is that the dialog gives the user the chance to amend the message before it is sent. A custom oversharing dialog contains information customized by the Microsoft 365 tenant. Figure 1 shows a custom oversharing dialog in Outlook (classic).
As you can see, the oversharing dialog gives the user the opportunity to override the block by providing a justification and acknowledging that the content meets organizational standards.
Outlook must have the opportunity to recognize that a violation is present, and it is possible that the oversharing dialog never appears if a user creates and sends a message quickly. You can configure a delay in sending to allow time for content evaluation against policies or let the message flow through to the transport pipeline. If the DLP evaluation in the transport pipeline detects a problem there, it will take whatever action is configured in the policy, such as blocking delivery of the message to external recipients.
Creating a Custom Oversharing Dialog
To implement a custom oversharing dialog, upload a JSON file to a policy rule after setting the “upload a JSON file containing custom content that will be used in the pop-up dialog” checkbox in the policy tips section of the rule configuration.
The JSON file contains the text for the custom directives in up to ten languages (a default language must be chosen). The text can include tokens for matched recipients, attachment file names, and sensitivity labels. The format and syntax of the JSON file is checked when the updated policy is submitted in the Purview portal.
Composing JSON can be a challenge if you’re not fluent in this subject. I usually start with the example file in the Microsoft documentation and make the necessary changes in Visual Studio Code (Figure 2).
A DLP rule can also be updated with the JSON to describe a custom oversharing dialog via PowerShell. To do this, put the content of the file in a variable and then use the Set-DlpComplianceRule cmdlet to update the rule. Before you can run the cmdlet, you’ll need to connect to the compliance endpoint after connecting to Exchange Online (run Connect-ExchangeOnline and then Connect-IPPSSession):
$JSONCustomContent = Get-Content "c:\temp\Rule.JSON" -Encoding UTF8 | Out-String Set-DlpComplianceRule -Identity "Oversharing Rule 1" -NotifyPolicyTipCustomDialog $JSONCustomContent -NotifyPolicyTipDisplayOption Dialog
Validation of the JSON content occurs when PowerShell attempts to update the rule. For example, DLP checks that the custom title property for each language does not exceed the 75-character maximum.
Like other changes to DLP policies, it can take a few hours to synchronize the update within a tenant. Clients must then pick up the updated policies before the changes are fully active. Both Outlook (classic) and the new Outlook support the same custom oversharing dialogs.
Oversharing Dialog Works
The oversharing dialog for DLP policies is a good idea that works well. Tweaking the JSON to create the desired effect for the dialog can take a few attempts and the limitations imposed by Microsoft are a little restrictive. But all-in-all, oversharing dialogs are a feature that should be considered for any DLP email-focused policy.
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