Microsoft 365 Signature Stamping

Microsoft 365 signature stamping applies an email signature to outgoing email at the Exchange Online mail server, not at the user's Outlook client. The stamping happens after the user sends the message but before the email leaves the organization, which means the signature is consistent whether the user composed the email from Outlook desktop, Outlook on the web, the Outlook mobile app, or any other client connected to the Microsoft 365 mailbox. Two technical implementations exist. The first is an Exchange Online Transport Rule (also known as a mail flow rule) that appends an HTML disclaimer to messages, configured through the Exchange admin center or PowerShell. The second is a Microsoft Outlook add-in installed organization-wide that injects the signature at compose or send time. The transport rule approach is universal but limits dynamic personalization. The add-in approach (such as Microsoft's own Outlook Roaming Signatures or third-party tools) supports per-user dynamic content but requires the user to have an active Outlook client.