MIME Content Access for Group Mailboxes via Microsoft Graph API

Hi @Suthan Raja 

Microsoft Graph currently does not provide full MIME access for Microsoft 365 Group conversations (posts), and this is an important distinction compared to user mailboxes.  

1.Does Microsoft Graph support retrieving full MIME content for Group Mailboxes?  

At this time, Microsoft Graph does not support retrieving full MIME content in Microsoft 365 Group conversations (threads/posts). MIME retrieval using the $value segment is only supported for message resources such as user mailbox messages. 

2.Is there an equivalent to /messages/{id}/$value for group posts? 

There is no equivalent endpoint in Microsoft Graph for: /groups/{group-id}/threads/{thread-id}/posts/{post-id} 

Group posts are exposed through the Graph API as structured objects (including properties such as body, from, receivedDateTime, and attachments) rather than as raw email messages. As a result, the original MIME format (including full transport headers and encoding details) is not available through these endpoints. 

3.Recommended approach to reconstruct MIME content 

Since MIME content is not directly available for group posts, there is no supported way to retrieve or fully reconstruct the original MIME message via Graph. 

For migration scenarios, the typical approach is:

-Extract the available structured data from the post (body content, sender, timestamps, attachments)

-Map these fields to a best‑effort message representation:

• body.content > message body (HTML)
• Attachments > retrieved via /attachments
• Metadata > map fields such as sender, subject (if available), and timestamp to standard headers (e.g., From, Subject, Date)

However, please note that:

-Original internet headers (Message-ID, Received) are not available.

-Exact MIME structure (multipart boundaries, encoding) cannot be reproduced.

-Transport-level details (such as DKIM or routing headers) are not exposed.

One partial exception is that if a group post contains an attached Outlook message, you can retrieve MIME for that specific attachment using:

GET /groups/{id}/threads/{id}/posts/{id}/attachments/{id}/$value

This applies only to attached messages, not to the group post itself.

References: 

• Get MIME content of a message using the Outlook mail API - Microsoft Graph | Microsoft Learn
• Get post - Microsoft Graph v1.0 | Microsoft Learn 

I hope this helps clarify current behavior. 

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