Classic Outlook Secondary Account Add-ins in New Outlook: What Changed

If you use multiple email accounts in Classic Outlook, you may have installed add-ins that work only with a secondary account. In New Outlook for Windows, the way add-ins load and interact with secondary accounts has changed. This article explains the technical differences, how add-in activation now works, and what you need to adjust for your secondary account add-ins to function correctly.

Why Add-in Behavior Changed Between Classic and New Outlook

Classic Outlook uses the MAPI subsystem and loads COM add-ins at the application level. When you added a secondary Exchange account, you could install a COM add-in that scanned that specific mailbox. The add-in had access to the active mailbox via the Application.ActiveExplorer.CurrentFolder.Store property.

New Outlook is built on the Outlook Web App (OWA) platform. It uses a single web-based rendering engine and does not support COM add-ins. All add-ins in New Outlook are web add-ins that run in a sandboxed browser frame. These add-ins register using an XML manifest that specifies which mailbox they should activate in, but the activation logic is tied to the user identity, not the MAPI store.

How Add-in Activation Works in Classic Outlook

In Classic Outlook, each account has its own set of add-ins. You can see this in File > Options > Add-ins. COM add-ins appear under Active Application Add-ins. If you installed an add-in for a secondary account, it loaded when that account was the active mailbox in the folder pane. The add-in could read the current mailbox context and perform actions like sending messages from that account.

How Add-in Activation Works in New Outlook

New Outlook loads add-ins based on the primary identity of the signed-in user. When you add a secondary account, the add-in manifest checks the AppDomains and Permissions elements. The add-in only activates if the secondary account belongs to the same Microsoft 365 tenant as the primary account. If the secondary account is a different tenant (for example, a personal Outlook.com account), the add-in does not receive the activation event for that mailbox.

Steps to Verify and Enable Secondary Account Add-ins in New Outlook

Before you start, confirm that your secondary account is in the same Microsoft 365 tenant as your primary account. If it is not, the add-in will not activate for that account.

1. Open New Outlook and go to settings
   Select the gear icon in the top-right corner. Choose Mail > General > Manage add-ins.
2. Check the add-in status for the secondary account
   In the Manage add-ins pane, look for the add-in you want to use with the secondary account. Click the three dots next to the add-in name and select Add-in details. Under Account availability, confirm that the secondary account is listed. If it is not, the add-in cannot be enabled for that account.
3. Enable the add-in for the secondary account
   If the secondary account appears in the list, toggle the switch to On. The add-in will now appear in the ribbon when you select the secondary account in the folder pane.
4. Test the add-in with the secondary account
   Click the secondary account in the folder pane. Open a message from that account. The add-in should appear in the ribbon. If it does not, restart New Outlook and repeat steps 1 through 3.

If Add-ins Still Do Not Work for a Secondary Account

Add-in is not listed in Manage add-ins for any account

This means the add-in is a COM add-in that was installed in Classic Outlook. New Outlook does not load COM add-ins. You must find a web add-in version of the same tool. Contact the add-in vendor and ask for an Office Add-in manifest file (XML). You can sideload it via the Add-ins for Outlook section in New Outlook settings.

Secondary account is not shown in Account availability

The secondary account is likely in a different tenant. New Outlook only allows add-in activation for accounts that share the same Exchange Online organization. To work around this, use the secondary account as the primary account by signing out of New Outlook and signing in with the secondary account first. Then add the original primary account as a secondary account.

Add-in loads but does not see the secondary mailbox

Some add-ins use the mailbox.getCallbackTokenAsync API to access the mailbox. In New Outlook, this API returns the token for the primary mailbox even when the user interface shows a secondary account. The add-in must be updated to use the mailbox.getUserIdentityTokenAsync API and check the accountType property. If the add-in vendor does not update it, the add-in will not function correctly for secondary accounts.

Item	Classic Outlook	New Outlook
Add-in type supported	COM add-ins and web add-ins	Web add-ins only (Office Add-ins)
Add-in activation scope	Per MAPI store (per mailbox)	Per user identity (primary account)
Secondary account support	Full, any account type	Only same-tenant secondary accounts
Add-in installation method	MSI, EXE, or XML manifest	AppSource or sideloaded XML manifest
API for mailbox context	Application.ActiveExplorer.CurrentFolder.Store	mailbox.getUserIdentityTokenAsync
COM add-in loading	Loaded at startup for all accounts	Not loaded at all

You can now identify why your secondary account add-ins do not work in New Outlook. Check whether the add-in is a COM add-in or a web add-in. If it is a COM add-in, find a web add-in replacement. If it is a web add-in but does not activate for the secondary account, verify that both accounts belong to the same Microsoft 365 tenant. For add-ins that load but ignore the secondary mailbox, ask the vendor to update the add-in to use the mailbox.getUserIdentityTokenAsync API. As an advanced step, you can use the secondary account as the primary account by signing out and signing in with that account first.