Outlook Add-In (Email Signatures)

An Outlook add-in for email signatures is an extension built on the Microsoft Office Add-In platform that runs inside Outlook (desktop, web, and mobile) and injects signatures into outgoing messages. Unlike Exchange transport rules which stamp signatures at the server, add-ins operate inside the user's mail client, which means they can read the current user's profile data, present interactive UI (such as letting the user choose between signature variants), and ensure the signature appears in the user's Sent Items folder exactly as the recipient sees it. Add-ins are distributed organization-wide through the Microsoft 365 admin center using centralized deployment, which installs the add-in for every targeted user without per-machine setup. Add-ins are required when the use case demands the signature be visible to the sender during compose, when multiple signature variants must be selectable per message, or when the signature includes dynamic elements that depend on user choice. The trade-off is that add-ins only run when the user composes from a supported Outlook client and require the add-in to be approved and deployed by an administrator.