Outlook Add-in

Outlook Add-ins are cross-platform, web-based extensions that integrate third-party services directly into your mail and calendar workflow, maximizing productivity.

Outlook Add-ins: These are web-based solutions (HTML, CSS, JavaScript) that extend your Microsoft Outlook client’s core functionality across Windows, Mac, Web, and Mobile platforms. They eliminate app-switching by embedding key features right into your inbox and calendar: for instance, integrating your Salesforce CRM data, automating email-to-task conversion with tools like MeisterTask, or streamlining meeting setup with Zoom or FindTime. The modern web add-in architecture ensures simple, centralized deployment for administrators and delivers a consistent, powerful experience for all 365 users.

https://learn.microsoft.com/en-us/office/dev/add-ins/outlook/outlook-add-ins-overview