Outlook Classic restricts Office.js network access

Question

Outlook Classic restricts Office.js network access

Aron Marinescu 0

Hello,

I am currently working on a Outlook add-in that applies the signature at the end of the body when the user starts composing an email or replies to another email.

Currently I have two javascript files, one is the taskpane.js and the other is the command.js file. Both of these make an API call to retrieve the current user's signatures (in HTML format).
The taskpane.js file is supposed to run when the user explicitly opens the application from the ribbon. The command.js file should run in the background.

Inside my manifest.xml I have the following values (frontend and backend values are placeholders):

1. Runtimes

<Runtimes>
  <Runtime resid="WebViewRuntime.Url">
    <Override type="javascript" resid="JSRuntime.Url" />
  </Runtime>
</Runtimes>

2. AppDomains

<AppDomains>
  <AppDomain>__FRONTEND_BASE_URL__</AppDomain>
  <AppDomain>__BACKEND_API_URL__</AppDomain>
</AppDomains>

3. LaunchEvent ExtensionPoint (Desktop)

<ExtensionPoint xsi:type="LaunchEvent">
  <LaunchEvents>
    <LaunchEvent Type="OnNewMessageCompose" FunctionName="onNewMessageCompose" />
    <LaunchEvent Type="OnMessageRecipientsChanged"
      FunctionName="onMessageRecipientsChanged" />
  </LaunchEvents>
  <SourceLocation resid="WebViewRuntime.Url" />
</ExtensionPoint>

4. Relevant Resource URLs

<bt:Url id="WebViewRuntime.Url"
  DefaultValue="__FRONTEND_BASE_URL__/commands.html" />
<bt:Url id="JSRuntime.Url"
  DefaultValue="__FRONTEND_BASE_URL__/commands.js" />

5. Permissions

<Permissions>ReadWriteItem</Permissions>

On Outlook Web, both the taskpane and the command scripts work fine, but on Outlook Classic only the taskpane can execute network calls.

Initially, when using the fetch() function, this error was returned: The request does not meet the requirements for Same-Origin policy or Simple Cross-Origin resource sharing.

So I proceeded to setup my .well-know/microsoft-officeaddins-allowed.json as this:

{
	"allowed": [
		"__FRONTEND_BASE_URL__/command.js"
	]
}

When monitoring network requests with Fiddle, I saw that Outlook classic successfully requested and downloaded the microsoft-officeaddins-allowed.json file.

After restarting Outlook classic, the command.js file gave me this error: A connection with the server could not be established.
Also, I noticed (by watching Fiddle logs) that the requests made by the command.js never leaved the machine.

I'm wondering if there is a way to give the background script network access in order to execute API calls.
Thank!

1 answer

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Answer 1

Kudos-Ng 14,515 Microsoft External Staff Moderator

Hi Aron Marinescu,

Thank you for posting your question in the Microsoft Q&A forum.

After researching this, I found many developers have reported the same behavior in Classic Outlook. The recurring explanation is the runtime difference, specifically: In classic Outlook on Windows, the LaunchEvent code runs in a JavaScript-only runtime instead of a webview browser control, which is why behavior differs from Outlook on the web (and from taskpane code).

However, Microsoft documents that the JavaScript-only runtime can request external data, but if your add-in operates in that runtime, you must use absolute URLs in fetch() calls because relative URLs aren’t supported there.

Reference: https://learn.microsoft.com/en-us/office/dev/add-ins/testing/runtimes

I hope the information above helpful.

Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.

Aron Marinescu 0 Reputation points

2026-02-27T16:20:19.15+00:00

Hello Kudos, thanks for the reply. In our application we do in fact use absolute urls (protocol + domain + path), but somehow the error "A connection with the server could not be established" still appears. It could suggest that outlook might be blocking network requests, but I'm not sure.
Kudos-Ng 14,515 Reputation points Microsoft External Staff Moderator

2026-03-02T10:45:45.7+00:00

Hi Aron Marinescu,

Thanks for the follow-up. After looking further, I unfortunately don’t have additional insights to add at this point.

I should also clarify that this is a user‑to‑user support forum. Moderators, contributors, and Microsoft employees participating here don’t have access to backend systems or the ability to directly intervene in product behavior. Our role is limited to sharing technical guidance and best practices based on reported issues and public documentation.

In this case, I also don’t have an environment where I can reliably reproduce or test this specific scenario in classic Outlook, so I’m unable to validate or dig deeper beyond what we’ve already discussed. Sorry about that.

That said, you may want to consider raising this on the GitHub discussions. That space tends to attract more add‑in developers with deep hands‑on experience, and there’s a good chance others have run into the same behavior and can share more concrete findings or confirmations.

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Outlook Classic restricts Office.js network access

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