‎ Microsoft Graph API

Question

API Endpoint: Request ID: <<removed PII>> Client Request ID: <<removed PII>>

Problem: We are creating a message in a shared mailbox from its raw MIME content, fetched from another user's mailbox. The API call is failing with a deserialization error, even though we are providing the data as a Blob with the correct Content-Type.

Answer 1

Dear Prasanna Suraweera

Thank you for posting your question on the Microsoft Q&A forum, and thank you to Emmanuel Santana for the initial response.

Based on my research, the “UnableToDeserializePostBody” error you encountered when calling POST /users/{sharedMailboxId}/messages is likely due to how the MIME content is included in the request body. Microsoft Graph requires the entire RFC 822 message to be base64-encoded and provided as a plain text string, with the Content-Type header set to text/plain.

If raw MIME bytes or a binary Blob are sent, the API attempts to interpret the body as Base64 and fails, resulting in the deserialization error. This often happens when MIME content is retrieved using GET /messages/{id}/$value (which returns raw bytes) and then posted without encoding. To resolve this, base64-encode the full MIME content before sending and ensure no extra characters or JSON wrappers are added.

For more details, please refer to the official documentation:

• Create message
• Send messages with MIME content

Please note that this is a suggestion and may not immediately resolve the issue. For a more accurate investigation, I would need additional details such as:

• The code snippet you are using to make the request
• The full error message (including any inner error details)
• How the code is being triggered (e.g., SDK, REST call, language)

Looking forward to your feedback.

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Answer 2

Hello. How exactly are you retrieving and formatting the MIME content from the source mailbox? Are you using /messages/{id}/$value or /messages/{id}/mimeContent to get it?