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How to Fix "An Unknown Issue Occurred While Sending Your Mail Merge" Error (Web Add-in)

This article helps you resolve the red banner error "An unknown issue occurred while sending your mail merge" in the Web Add-in.

Work through each step in order. After each step, click Send & Track again to test.

Step 1: Use the correct shared mailbox workflow

If you're sending from a shared mailbox, Outlook can create drafts in the wrong location. Follow this workflow instead.

If this used to work and recently stopped: Remove the shared mailbox from Outlook, re-add it, then retry.

Step 2: Test whether the error is draft-specific

If the error only happens with one draft or template:

  1. Create a new draft in ContactMonkey with simple text
  2. Insert the draft into a new Outlook message
  3. Add one internal recipient (for example, your own email address)
  4. Click Send & Track
  5. If that sends successfully, add your original content back in sections until the error reappears

This identifies whether a specific content block is triggering the issue.

Step 3: Switch your expansion Method

The expansion method controls how ContactMonkey processes your recipient list (Exchange Admin or Graph API). Switching methods can resolve send failures caused by authentication or connectivity conflicts.

  1. Open the Web Add-in
  2. Click the gear icon above Send & Track
  3. Under Default expansion method, switch between Exchange Admin - Backend and Graph - Backend
  4. Click Home to return to the previous page
  5. Click Send & Track

Step 4: Remove special characters from your sender name

If you're sending from an alias, accented or special characters in the sender name can trigger this error. Ensure the name only contains alphanumeric characters before retrying.

Step 5: Shorten your preheader text

Outlook imposes a 130-character maximum for preheader text. If you saved preheader text in the Email Builder, you must shorten it before resending.

  1. Open your draft in the Email Builder
  2. Click the Settings tab at the top right
  3. Update the text in the Preheader field

Still getting the error? Contact Support

Collect the following before reaching out to support@contactmonkey.com — this helps Support investigate without back-and-forth:

  • A screenshot of the red error banner
  • Your recipient method: CSV upload, To field recipients, or distribution list
  • Whether you're sending from a shared mailbox
  • Your Outlook environment: Classic Outlook, Outlook on the web (OWA), or Outlook for Mac
  • The email subject line and approximate time the error occurred

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