When creating a meeting using "Meeting Room Mailbox", outlook 2021 cannot be forwarded.

Hi yanhaowen

Thank you for reaching out to Microsoft Q&A forum

As far as I know, the core difference stems from how meetings are created and processed in resource mailboxes like Meeting Room Mailboxes, which often leverage EWS for booking and calendar operations. EWS-generated meeting items in Exchange 2016 may lack or have incomplete extended properties. In contrast:

Outlook 2016 is more lenient in validating these properties, allowing forwarding to succeed without the "save first" prompt.

Standard user mailboxes create meetings via MAPI (Messaging Application Programming Interface) directly in Outlook, which sets all necessary properties correctly, enabling forwarding in both versions.

This discrepancy arises because resource mailboxes (e.g., rooms) use server-side calendar processing agents that can invoke EWS for automation, such as auto-accepting bookings or availability checks, resulting in items that appear "unsaved" or incomplete to Outlook 2021's stricter parsing.

Microsoft documentation confirms that EWS is the primary protocol for programmatic creation, modification, and booking of appointments in Exchange, including interactions with resource mailboxes like rooms, as stated in Create a meeting by using EWS. For example, when scheduling a meeting with a room, Outlook may use EWS to query availability and process the booking on the server side, embedding EWS-specific item creation.

Therefore, if you want to find workaround approaches for this context:

Method 1: Save the Meeting Before Forwarding

This is the simplest workaround for end users. Open the meeting item by double-clicking it in your calendar, then click File and select Save (or press Ctrl+S). Close the meeting window and then attempt to forward it again. This action forces Outlook 2021 to validate and complete the item's MAPI properties, allowing the forward operation to succeed.

Method 2: Use Online Mode Instead of Cached Mode

Try switching Outlook to Online Mode temporarily to determine if the issue is related to cached data synchronization.

• Go to File -> Account Settings
• Select your Exchange account, click Change
• Uncheck "Use Cached Exchange Mode." Restart Outlook and test if you can forward the meeting.

If the issue is reproduced with online mode and Outlook 2021 latest version, there could be some property issues for these emails created by EWS. If so, you can use MFCMAPI to check and contrast MAPI properties between normal meeting created by Outlook and the problematic one created by EWS.

Here are links to download and use MFCMAPI:

1. Release MFCMAPI December 2024 (24.0.24362.01) · microsoft/mfcmapi · GitHub
2. How to use MFCMAPI - A complete guide to MFCMAPI tool

Note: Microsoft is providing this information as a convenience to you. The sites are not controlled by Microsoft. Microsoft cannot make any representations regarding the quality, safety, or suitability of any software or information found there. Please make sure that you completely understand the risk before retrieving any suggestions from the above link.

Please feel free to leave a comment below if you require any additional help. 

Best regards

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