Why is my Outlook Add-in silently failing the "onMessageSendHandler" action when side-loading from a static website?

I've created a small Outlook Add-in using the Yeoman generator and I've developing it using Visual Studio Code.

The add-in only does two things:

1. When the onNewMessageComposeHandler action fires, it sets the email message's subject line to "Hello, World!"
2. When the onMessageSendHandler action fires, it adds an email address to the To: list of recipients.

This is the code in my launchevents.js file:

const bccRecipientEmail = "foo@bar.test";

function onMessageSendHandler(event) {
  setBcc(event);
  event.completed({ allowEvent: true });
}

function onNewMessageComposeHandler(event) {
  setSubject(event);
}

function setBcc(event) {
  Office.context.mailbox.item.bcc.addAsync(
    [bccRecipientEmail],
    {
      asyncContext: event,
    },
    function (asyncResult) {
      // Handle success or error.
      if (asyncResult.status !== Office.AsyncResultStatus.Succeeded) {
        console.error("Failed to set bcc to '" + bccRecipientEmail + "': " + JSON.stringify(asyncResult.error));
      }

      // Call event.completed() after all work is done.
      asyncResult.asyncContext.completed();
    }
  );
}

function setSubject(event) {
  Office.context.mailbox.item.subject.setAsync(
    "Set by an event-based add-in! WITH COMPOSE",
    {
      asyncContext: event,
    },
    function (asyncResult) {
      // Handle success or error.
      if (asyncResult.status !== Office.AsyncResultStatus.Succeeded) {
        console.error("Failed to set subject: " + JSON.stringify(asyncResult.error));
      }

      // Call event.completed() after all work is done.
      asyncResult.asyncContext.completed();
    }
  );
}

if (Office.context.platform === Office.PlatformType.PC || Office.context.platform == null) {
  Office.actions.associate("onMessageSendHandler", onMessageSendHandler);
  Office.actions.associate("onNewMessageComposeHandler", onNewMessageComposeHandler);
}

These are the relevant parts from my manifest.xml file:

<VersionOverrides xmlns="http://schemas.microsoft.com/office/mailappversionoverrides" xsi:type="VersionOverridesV1_0">
  <VersionOverrides xmlns="http://schemas.microsoft.com/office/mailappversionoverrides/1.1" xsi:type="VersionOverridesV1_1">
    <Requirements>
      <bt:Sets DefaultMinVersion="1.12">
        <bt:Set Name="Mailbox" />
      </bt:Sets>
    </Requirements>
    <Hosts>
      <Host xsi:type="MailHost">
        <!-- Event-based activation happens in a lightweight runtime.-->
        <Runtimes>
          <!-- HTML file including reference to or inline JavaScript event handlers.
               This is used by Outlook on the web and Outlook on the new Mac UI. -->
          <Runtime resid="WebViewRuntime.Url">
            <!-- JavaScript file containing event handlers. This is used by Outlook on Windows. -->
            <Override type="javascript" resid="JSRuntime.Url"/>
          </Runtime>
        </Runtimes>
        <DesktopFormFactor>
          <FunctionFile resid="Commands.Url" />
          <ExtensionPoint xsi:type="MessageReadCommandSurface">
		    [...]
          </ExtensionPoint>

          <!-- Can configure other command surface extension points for add-in command support. -->

          <!-- Enable launching the add-in on the included events. -->
          <ExtensionPoint xsi:type="LaunchEvent">
            <LaunchEvents>
              <LaunchEvent Type="OnNewMessageCompose" FunctionName="onNewMessageComposeHandler"/>
              <LaunchEvent Type="OnMessageSend" FunctionName="onMessageSendHandler" SendMode="PromptUser" />
            </LaunchEvents>
            <!-- Identifies the runtime to be used (also referenced by the Runtime element). -->
            <SourceLocation resid="WebViewRuntime.Url"/>
          </ExtensionPoint>
        </DesktopFormFactor>
      </Host>
    </Hosts>
    <Resources>
      <bt:Images>
        <bt:Image id="Icon.16x16" DefaultValue="https://[REDACTED].web.core.windows.net/assets/icon-16.png"/>
        <bt:Image id="Icon.32x32" DefaultValue="https://[REDACTED].web.core.windows.net/assets/icon-32.png"/>
        <bt:Image id="Icon.80x80" DefaultValue="https://[REDACTED].web.core.windows.net/assets/icon-80.png"/>
      </bt:Images>
      <bt:Urls>
        <bt:Url id="Commands.Url" DefaultValue="https://[REDACTED].web.core.windows.net/commands.html" />
        <bt:Url id="Taskpane.Url" DefaultValue="https://[REDACTED].web.core.windows.net/taskpane.html" />
        <bt:Url id="WebViewRuntime.Url" DefaultValue="https://[REDACTED].web.core.windows.net/commands.html" />
        <!-- Entry needed for Outlook on Windows. -->
        <bt:Url id="JSRuntime.Url" DefaultValue="https://[REDACTED].web.core.windows.net/launchevent.js" />
      </bt:Urls>
      <bt:ShortStrings>
        <bt:String id="GroupLabel" DefaultValue="Sync Emails to Quickbase"/>
        <bt:String id="TaskpaneButton.Label" DefaultValue="Show Taskpane"/>
        <bt:String id="ActionButton.Label" DefaultValue="Perform an action"/>
      </bt:ShortStrings>
      <bt:LongStrings>
        <bt:String id="TaskpaneButton.Tooltip" DefaultValue="Opens a pane displaying all available properties."/>
        <bt:String id="ActionButton.Tooltip" DefaultValue="Perform an action when clicked."/>
      </bt:LongStrings>
    </Resources>
  </VersionOverrides>
</VersionOverrides>

When I run the add-in on my machine and side-load it into Outlook on Windows¹, it works well. I can see the subject line change when I compose the message and when I send it, the email address is indeed added to the recipients list.

Now I'm trying to publish my add-in to a static site in Azure Storage, as per the documentation. When I side-load the add-in from the static site's URL I find that the subject line changes successfully but the email address is not added to the list of recipients.

My question is - Why, when I side-load the add-in from a static site, does the onMessageSendHandler not work as expected?

¹ Outlook version: Microsoft® Outlook® for Microsoft 365 MSO (Version 2304 Build 16.0.16327.20200) 64-bit

Try to add any logging statements to the events handlers, so you could see whether a particular event handler is invoked for sure.

Make sure that your code includes the following piece:

// IMPORTANT: To ensure your add-in is supported in the Outlook client on Windows, remember to map the event handler name specified in the manifest's LaunchEvent element to its JavaScript counterpart.
// 1st parameter: FunctionName of LaunchEvent in the manifest; 2nd parameter: Its implementation in this .js file.
if (Office.context.platform === Office.PlatformType.PC || Office.context.platform == null) {
  Office.actions.associate("onNewMessageComposeHandler", onNewMessageComposeHandler);
  Office.actions.associate("OnMessageSend", onMessageSendHandler);
}

Event-based add-ins running in Outlook on Windows don't run code included in the Office.onReady() and Office.initialize functions. We recommend adding your add-in startup logic, such as checking the user's Outlook version, to your event handlers instead.

See Implement event handling for more information about that.

Also try to find the add-in's bundle.js file, navigate to the following folder in File Explorer. Replace text enclosed in [] with your applicable Outlook and add-in information.

%LOCALAPPDATA%\Microsoft\Office.0\Wef\{[Outlook profile GUID]}\[Outlook mail account encoding]\Javascript\[Add-in ID]_[Add-in Version]_[locale]

Open bundle.js in Visual Studio Code and check the source code.

In the source code I've noticed that the completed method is called from the setAsync callback. That is not required at all. See Get, set, or add recipients when composing an appointment or message in Outlook for more information how to set up recipients.