Add-in Errors in classic desktop Outlook

[digiofficerobin]

When I create a new e-mail I get all kinds of add-in error taskpane while the addin is working fine in New Outlook Desktop and web.

Your Environment

• Platform : Windows PC 11
• Host : (classic) Microsoft® Outlook® for Microsoft 365 MSO (Version 2601 Build 16.0.19628.20132) 64-bit
• Office version number: 365
• Operating System: Windows 11

Expected behavior

We expect that if we develop an add-in for New Outlook, that it also works in classic Outlook because they both share the same minimum version API requirements. See table in article Outlook client support

Current behavior

When we start classic Outlook for Windows desktop and compose a new e-mail, then multiple taskpanes for the same add in are raised with error messages, and even a taskpane is shown which are from another app from another mailbox.

When we try to send an e-mail, then Outlook is waiting for the add-in to completes its task (which it never does) and users can only cancel the action without sending the e-mail. Thus users cannot even send e-mails anymore.

If the add-in should work (according to the minimum version requirements) also in classic Outlook for desktop, then we expect that there should be no differences.

If the add-in probably cannot work in classic Outlook, then (for our situation) it will be oke that the add-in doesn't load/start and stays silent. The main goal for us is to write an add-in that works in the New Outlook (desktop and web), but we cannot/will not prevent users to start classic Outlook as well.

As a workaround we use the EquivalentAddins override, but for this to work we still have to install our old add-in on all installations.

		  <EquivalentAddins>
			<EquivalentAddin>
			  <ProgId>NameOfMyCOMAddin</ProgId>
			  <Type>COM</Type>
			</EquivalentAddin>
		  </EquivalentAddins>

Steps to reproduce

1. Create an add-in for New Outlook with a button in the toolbar and a task pane
2. Make sure it works correctly in New Outlook
3. Start classic Outlook for Windows
4. Try to use the add-in

Link to live example(s)

See video:

Outlook-Add-In-Errors.mp4

Context

As far as we know, there is no way to debug the add-in or start Web Developer tools for classic Outlook for Windows.

Another developer has noticed the following differences as well:

• Event-based activation doesn't seem to work. So, I immediately create a branded email (the task pane opens with a yellow error).
• Dialog windows don't respect the width of the client (Outlook Classic app). The full screen width is used for the dialog.
• Adding attachments isn't possible. In One Outlook and the web version, these attachments are retrieved as blobs via an authenticated service request. Classic Outlook doesn't support blob URLs. The solution/workaround is to encode them to base64.
• Adding recipients throws exceptions. Classic Outlook expects an object containing not only the email address but also the displayName.
• The width of the task pane is customizable in classic Outlook, while it has a fixed width in New Outlook. Our layout should then be developed with responsive design.

Useful logs

Event-log entries:
Add-in Error
We can't start this add-in because it isn't set up properly.
P1: Apps for Office
P2: 16.0.19628.20132
P3: 0x8004323E
P4:

Failed to parse element: VersionOverrides
Id=bc13b9d0-5ba2-446a-956b-c583bdc94d5e, DisplayName=Suggested Meetings, Provider=Microsoft, StoreType=EXCatalog, StoreId=(null)
P1: Apps for Office
P2: 16.0.19628.20132
P3: 0x8004323E
P4: New Document

Failed to parse element: VersionOverrides
Id=7a774f0c-7a6f-11e0-85ad-07fb4824019b, DisplayName=Bing Maps, Provider=Microsoft, StoreType=EXCatalog, StoreId=(null)
P1: Apps for Office
P2: 16.0.19628.20132
P3: 0x8004323E
P4: New Document

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[github-actions]

Here are some similar issues that might help you. Please check if they can solve your problem.

• Addin not loading in new outlook client #3865
• Web add-in not loading in new Outlook windows version #4615

---

Possible solution (Extracted from existing issue, might be incorrect; please verify carefully)

Solution 1:

We conducted a further analysis and discovered that the issue lies with SAML authentication (using Microsoft Entra ID) for our app.

When using SSO with certificates, the add-in form appears correctly on the right task pane. Additionally, we noticed in the developer tools that with SAML, there are numerous redirects and authentication loops.

I have shared the .HAR file from a private repository for your review.

Reference:

• Web add-in not loading in new Outlook windows version #4615 (comment)

Solution 2:

This is the wiki on collecting logs - https://github.com/OfficeDev/office-js/wiki/Collecting-log-files-for-Outlook-Add%E2%80%90ins-issues

Reference:

• Web add-in not loading in new Outlook windows version #4615 (comment)

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[victoria-msft]

Hello @digiofficerobin,

Thank you for reaching out to us for assistance with this issue. Can you please provide us with additional information regarding your manifest?

Thank you,
Victoria

[digiofficerobin]

@victoria-msft I've attached a masked version of our manifest
OneOutlookManifest.xml

[timwan10]

You have this in your manifest:

<Runtime resid="residUILessFunctionFileUrl">
<!--  JavaScript file containing event handlers. This is used by classic Outlook on Windows.  -->
<!--  <Override type="javascript" resid="JSRuntime.Url"/>  -->
</Runtime>

<!--  Entry needed for Outlook Desktop .  -->
<!--  <bt:Url id="JSRuntime.Url" DefaultValue="https://digioffice365onlinelocal.xxxxxxxx.nl/events.js" />  -->

The part that is needed by Classic Outlook on Windows, has been commented out.

Please also note, that in New Outlook / OWA, Launch Events run in a web view, whereas in Classic Outlook it uses as JS Engine for performance reasons. There are many things that are not compatible.

These links may help:

https://learn.microsoft.com/en-us/office/dev/add-ins/develop/event-based-activation
https://learn.microsoft.com/en-us/office/dev/add-ins/testing/troubleshoot-event-based-and-spam-reporting-add-ins

[digiofficerobin]

We have made some progress in making our app workable and how to debug the event-based add-in. Nonetheless we think it is not worthwhile to invest much time in developing the add-in for classic Outlook, because classic Outlook is closing down in April 2026 https://windowsforum.com/threads/microsoft-to-sunset-classic-outlook-what-users-need-to-know.347817/ .

Is there a simple way to prevent our add-in to load in classic Outlook?

[ndeleuze]

@digiofficerobin, Outlook Add-ins are inherently cross-client, so there is no way to prevent loading for only some clients. Since classic Outlook fully sunsets in 2029, it may be best to work on fully supporting this platform as well. If you are set on not supporting classic, then the suggestion would be to handle this logic within the Add-in (i.e. check for platform before performing actions and when displaying taskpane to display some kind of error message to the user).

Note: The EquivalentAddins feature would suppress your Add-in in Classic Outlook if you have a specific, equivalent COM Add-in installed, which may not be helpful for your scenario.

I am closing this thread for since there is no bug to report.