Reply to email fails on Microsoft 365 Group mailbox (New Outlook/OWA) — ErrorItemNotFound on Groups export API

Question

Environment: Microsoft 365, Exchange Online, Modern/"New" Outlook and Outlook Web Access (OWA). Issue does NOT occur in Outlook Classic (desktop).

Problem:

For the last several days, replying to any email received on one of our Microsoft 365 Group mailboxes (public group with Teams, GroupMailbox type) fails in New Outlook and OWA. The send fails with an internal server error.

Exact error shown:

"There was a problem sending this message. Please try again later. An internal server error occurred."

Error details:

[Message]: "The specified object was not found in the store."

StatusCode: NotFound

Method: GET

Url: https://outlook.office.com/api/beta/Users('<user-guid>')/Groups('<group-guid>@<tenant-guid>')/Messages('<item-id>')/export

Also shown: "The original message can't be shown right now. It will be included when this message is sent."

What we've already checked/ruled out:

1. Permissions — confirmed via Exchange Online PowerShell that the affected users have FullAccess on the group mailbox and are listed correctly in GrantSendOnBehalfTo and have SendAs rights.
2. Message delivery/transport — Get-MessageTraceV2 shows prior successful sends (via Outlook mobile) as "Delivered" with no failures around the same time window.
3. Client-side cache/auth — cleared cookies, removed and re-added the group mailbox in Outlook Web, updated the browser, signed out/in fully. No change.
4. Confirmed workaround — replying from Outlook Classic and Mobile application (desktop, MAPI-based) works without any error. Only New Outlook / OWA fail.

Based on the error (GET .../Groups/.../Messages/.../export returning 404 ErrorItemNotFound), it looks like New Outlook/OWA is unable to fetch the original message from the Group mailbox's own store when composing a reply, even though the message displays fine in the UI. This suggests a backend issue specific to how New Outlook/OWA retrieves items from M365 Group mailboxes for the reply/export flow.

Questions:

1. Is this a known issue with the Groups REST "export" endpoint used by New Outlook/OWA for Group mailbox replies?
2. Is there a tenant-side or service-side fix, or is this purely a Microsoft backend bug we need to wait on?
3. Any timeline or workaround beyond using Outlook Classic?

We are Global Admins on the tenant and can provide further diagnostic output (message trace, audit logs, full error payload) if needed.

Answer 1

Hi @Jayesh Parmar

Currently, I have not found any published Service Health advisory or Known Issue that specifically matches the ErrorItemNotFound error returned by the Groups export endpoint. Based on your description, the issue only occurs in New Outlook and Outlook on the web (OWA), while Outlook Classic and Outlook Mobile continue to work successfully. This suggests the issue may be related to the service components used by the modern Outlook experience rather than mailbox permissions, mail flow, or client configuration

To investigate this further, I would recommend opening a support ticket and providing the Request ID, Correlation ID, timestamp (UTC), and network trace so Microsoft can review the backend logs associated with the failed /Groups/.../Messages/.../export requests. A Microsoft Support Engineer can perform a remote session to investigate the issue, collect additional diagnostic information, validate the mailbox and service-side configuration, and perform any necessary backend checks. If the issue requires further attention, they can escalate the case to the appropriate engineering team for deeper analysis and resolution.

Reference: Get support - Microsoft 365 admin | Microsoft Learn 

Note: To create a support ticket, you need to have Microsoft 365 admin privileges and if you aren’t admin of your organization, please contact the admin of creating a new ticket. 

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