Can New Outlook Replace Classic Outlook for COM Add-ins? Practical Answer

Many business users rely on COM add-ins to extend Outlook functionality with tools for CRM, email archiving, compliance, or productivity. Classic Outlook supports COM add-ins natively, but the new Outlook for Windows uses a different architecture that does not load COM-based extensions. This article explains the technical differences between the two Outlook versions regarding COM add-in support, what currently works, and what does not. You will learn whether you can switch to new Outlook without losing your existing add-ins and what alternatives are available.

Why New Outlook Cannot Load COM Add-ins

Classic Outlook (the desktop version included with Microsoft 365 and Office 2021/2019) is a native Windows application built on MAPI and the Windows Component Object Model. COM add-ins are compiled DLL or EXE files that register with Outlook and run in-process with the same memory space. They have deep access to the Outlook object model, allowing them to modify UI elements, intercept events, and process data in real time.

New Outlook for Windows is a progressive web app built on the Outlook Web App (OWA) platform. It uses a web rendering engine and runs add-ins from the Office Add-ins platform, which are sandboxed web applications. This architecture does not include a COM runtime or MAPI stack, so it cannot load compiled COM components. Microsoft designed new Outlook to be faster, lighter, and more secure, but this means sacrificing backward compatibility with legacy COM add-ins.

Technical Root Cause

The core difference is the execution environment. Classic Outlook runs as a 32-bit or 64-bit native process that can call into COM libraries. New Outlook runs inside a web container that only supports JavaScript APIs. COM add-ins require direct access to the Outlook object model through interfaces like IDTExtensibility2, which are not available in the web platform. Microsoft has not and will not backport COM support to new Outlook because the entire architecture would need to change.

Steps to Check If Your COM Add-in Works in New Outlook

Before migrating to new Outlook, you must verify that each COM add-in you use has a web-based replacement. Follow the steps below to inventory your add-ins and test compatibility.

1. Open Classic Outlook and list your COM add-ins
   Go to File > Options > Add-ins. In the Manage dropdown, select COM Add-ins and click Go. Write down the name and version of every add-in listed.
2. Check the add-in vendor’s documentation
   Visit the vendor website or contact support. Ask whether the add-in is compatible with new Outlook for Windows. Many vendors now offer a web add-in version or have announced a migration timeline.
3. Enable new Outlook and test each add-in
   Click the toggle in the top-right corner of Classic Outlook to switch to new Outlook. Look for the add-in in the ribbon or in the Add-ins pane (Home > Get Add-ins). If the add-in does not appear, it is not supported.
4. Search the Office Add-ins store for an alternative
   In new Outlook, go to Home > Get Add-ins. Search for the add-in by name or function. If a web version exists, install it and test its features.
5. Switch back to Classic Outlook if needed
   If the required add-in is missing, click the toggle again and select Classic Outlook. Your COM add-in will resume working immediately.

If Your COM Add-in Is Not Supported in New Outlook

Most legacy COM add-ins from small vendors or custom in-house development will never be ported to the web add-in platform. In these cases, you have three options.

Stay on Classic Outlook Indefinitely

Microsoft has not announced a retirement date for Classic Outlook. You can continue using it with all your COM add-ins. New Outlook is optional and can be turned off by IT administrators via group policy. Classic Outlook will receive security updates through at least 2028 for Microsoft 365 subscribers.

Request a Web Add-in from the Vendor

If the add-in is business-critical, ask the vendor to develop a web add-in version. The Office Add-ins platform uses a manifest XML file and JavaScript, which is easier to deploy and update than COM add-ins. Some vendors offer both versions during the transition period.

Build a Custom Web Add-in

If your organization developed a custom COM add-in in-house, you can rebuild it as a web add-in using the Office JavaScript API. The API supports most common Outlook operations: reading and writing mail items, creating appointments, and adding custom buttons to the ribbon. Start with the Microsoft documentation for Office Add-ins and the yeoman generator for Outlook add-in projects.

Item	Classic Outlook	New Outlook
COM add-in support	Full support for DLL and EXE add-ins	No support; COM add-ins do not load
Add-in platform	COM (Component Object Model)	Office Add-ins (web-based JavaScript)
Performance impact of add-ins	Can slow down Outlook if poorly coded	Sandboxed; less impact on stability
Deployment method	MSI installer or regsvr32 registration	Centralized manifest via SharePoint or AppSource
Custom ribbon buttons	Via IRibbonUI callbacks in COM code	Via VersionOverrides in add-in manifest
Offline capability	Full offline access to mailbox	Limited; requires cached mode and sync

New Outlook cannot replace Classic Outlook for any COM add-in that has not been rewritten as a web add-in. The two platforms are architecturally incompatible. You can use both versions side by side by toggling between them, but you will lose all COM add-in functionality in new Outlook. For organizations with a heavy investment in COM add-ins, staying on Classic Outlook is the only practical choice until vendors provide web-based alternatives. If you are starting fresh or planning a migration, consider the Office Add-ins platform as the future-proof replacement for COM add-ins.