OnSend Event Triggered Multiple Times in Loop - Outlook for Mac 16.105 (Latest) - Without New Editor

[harshitgoyal1711]

OnSend Event Triggered Multiple Times in Loop - Outlook for Mac 16.105 (Latest) - Without New Editor

Environment

Field	Value
Platform	Outlook for Mac
Outlook Version	16.105 (26011018) - Released January 13, 2026
OS	macOS
Feature	Office Add-in (OnSend Event)
New Editor	Disabled

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Issue Summary

The OnSend event is being triggered multiple times in a loop when sending certain emails. This behavior is similar to #6376, but occurs without the New Editor enabled.

Multiple customers are reporting this issue on the latest Outlook for Mac version.

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Observed Behavior

1. User composes a Reply/Forward email in Outlook Mac
2. The OnSend add-in executes and completes successfully
3. Outlook triggers the OnSend event again after mail is sent
4. This causes the add-in to execute multiple times in a loop
5. Result: Classification/content is added multiple times to the email body

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Key Observations

• Issue occurs without "New Editor" option enabled (unlike OnSend Event Triggered Multiple Times in Loop When New Editor is Enabled - Outlook for Mac 16.103.4 #6376)
• Issue is specific to certain mail threads, not all emails
• Issue appears in Reply/Forward scenarios with existing classification headers
• Add-in logs confirm the process completes successfully before being re-invoked
• The re-invocation appears to be initiated by Outlook, not by the add-in
• Same issue NOT observed when copy-pasting the same mail thread body into a freshly composed email
• Multiple customers are experiencing this issue

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Expected Behavior

The OnSend event should be triggered only once per send action. After the add-in completes execution and calls event.completed({ allowEvent: true }), Outlook should not re-invoke the OnSend event.

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Workaround Tested

• ✅ Copy-pasting the mail thread body to a freshly composed email prevents the loop
• ❌ No other workaround available currently

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Diagnostic Information

Incident ID: BPMKV97Y

Runtime logs are available upon request.

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Questions for Microsoft

1. Why is the OnSend event being re-triggered after event.completed({ allowEvent: true }) is called?
2. Is this related to the same root cause as OnSend Event Triggered Multiple Times in Loop When New Editor is Enabled - Outlook for Mac 16.103.4 #6376, or a separate issue?
3. Why does this occur only on specific mail threads (Reply/Forward with existing headers)?
4. Is there a fix or patch planned for this behavior?
5. Are there any recommended workarounds or manifest/code changes to prevent this re-invocation?
6. Why does the issue not reproduce when the same content is pasted into a new compose window?

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Related Issue

• #6376 - OnSend Event Triggered Multiple Times in Loop When New Editor is Enabled

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Requested Action

Please investigate why Outlook is re-triggering the OnSend event after the add-in has completed execution on the latest Outlook for Mac version (16.105), specifically for Reply/Forward scenarios.

Incident ID for diagnosis: BPMKV97Y

Please let us know if you need additional logs or information to assist with the investigation.

[github-actions]

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

• Outlook Onsend addin is invoked multiple times after clicking send #1367
• Outlook Onsend addin is invoked multiple times after clicking send #5520

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

Solution 1:

Please provide the manifest URL of the add-in you are using. This will help us investigate the issue further.

Reference:

• Outlook Onsend addin is invoked multiple times after clicking send #1367 (comment)

Solution 2:

Please share the exact build number of the build you see this issue in. Is it possible for you to share a test account where this issue is reproducible?

Reference:

• Outlook Onsend addin is invoked multiple times after clicking send #1367 (comment)

Solution 3:

Please provide logs from Outlook using the below steps:

1. Launch Outlook (start a new session)
2. Wait for Add-ins to load
3. Reproduce the On-Send issue
4. Click on Help -> Collect Diagnostics.
   This will generate an Incident ID. Note the same and share it with admin@M365x010952.onmicrosoft.com.

Reference:

• Outlook Onsend addin is invoked multiple times after clicking send #1367 (comment)

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[harshitgoyal1711]

@exextoc Can you please look into this ?

[rishi0107]

Hi @exextoc We have encountered one more case as such
Incident ID: TPYE5FZB, logs can be provided upon request.