How do I fix the 'unable to deserialize' error when reattaching Outlook PDF attachments using n8n?

Below is some guidance on troubleshooting the "unable to deserialize" error for attachments when creating a draft in MS Outlook via n8n.

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What the Error Means

The error message indicates that the node expected to receive binary data in a specific format, but what it received couldn’t be properly interpreted (or “deserialized”). In other words, the attachment data isn’t being passed in a way that the node recognizes.

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Steps to Troubleshoot and Fix

1. Verify the Binary Data Format
   Make sure that when you retrieve the email attachments, the binary data is intact and is in the format expected by n8n. This means:

   • The attachment should be available as a binary property (e.g. on the item’s binary object).
   • Its value should be a valid base64-encoded string along with the proper metadata (such as fileName and mimeType).

2. Check the Attachment Field Name
   In the node that creates the new draft, ensure that the field you specify for attachments (often via an option like “Attachment Field Name” or similar) exactly matches the name of the binary property produced upstream.

   • If the binary data field name is different from what the draft node expects, it won’t find the attachment—and may trigger a deserialization error.

3. Confirm Binary Property Passing
   When passing data between nodes:

   • Use a Set or Function node to inspect the item structure.
   • Confirm that the binary part of the item contains the attachment with proper keys (for example, data, mimeType, and fileName).

4. Convert or Reformat if Needed
   If the attachment was retrieved from another source and is not in the expected structure, consider converting it using a Function node to construct the binary property yourself. An example snippet:

   JSCopied
   // Example: Ensure that the attachment is in the correct structure.
   if (items[0].binary && items[0].binary.myAttachment) {
     items[0].binary.myAttachment = {
       data: items[0].binary.myAttachment.data, // base64 string
       mimeType: items[0].binary.myAttachment.mimeType, // e.g. "application/pdf"
       fileName: items[0].binary.myAttachment.fileName, // e.g. "document.pdf"
     };
   }
   return items;

5. Double-check Node Options for Draft Creation
   Although the documentation for the Outlook node does not extensively describe attachment handling, the process is similar to other nodes (like Gmail) where you must provide the correct binary property name. Make sure the option for adding an attachment is enabled and configured with the correct attachment field name.

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Final Thoughts

The key is to ensure that the attachment data is passed as a correctly structured binary object. Verifying the data structure from the retrieval node and matching it with the expected input of the draft node should help resolve the “unable to deserialize” error.

If after these checks the error still persists, you might want to:

• Revisit the settings and credentials in your Outlook node to rule out any configuration issues.
• Use a debugging node (e.g. the Function node with logging) to output the exact structure of your item’s binary data.

This should guide you to pinpoint where the data format is off and how to correct it.