Customising the 'Select a label' tooltip for Outlook

You're right that the default tooltip in Outlook — “Select a label - Your organization requires you to label this email before sending” — cannot be directly customized, as it's hardcoded and not affected by label-level descriptions in Microsoft Purview.

However, there is a workaround that allows you to present a customized message to the user by leveraging a Data Loss Prevention (DLP) policy in Exchange.

You can create a custom DLP rule in the Microsoft Purview compliance portal targeting Exchange email that detects when an email or its attachments are not labeled with a sensitivity label. When this condition is met, you can configure a custom policy tip with wording tailored to your communication plan, tone of voice, and preferred spelling (e.g., British English).

Important: For policy tips to appear natively in Outlook (both desktop and web), users must be licensed with Microsoft 365 E5, Microsoft 365 E5 Compliance, or an equivalent add-on that includes DLP policy tips in Outlook.