Unable to send messages as a Microsoft 365 Group

Alexsander Linn 40 Reputation points
2026-08-12T18:37:42.5966667+00:00

Hello,

One of our employees is experiencing an issue when trying to send an e-mail as a Microsoft 365 Group.

When she selects the group as the sender and tries to send the message, the following error appears:

"Illegal cross server connection. Cannot open mailbox."

I have already tried removing and reassigning the Send As permissions, changing the permissions, and testing again, but the issue persists.

Initially, the user had both Send As and Send on Behalf permissions. I removed the Send on Behalf permission and left only Send As, but the issue remains the same. And vice versa.

The user is able to access the group normally, and the problem occurs specifically when trying to send a message as the group.

Could you please help us investigate this issue?

Thank you.

Exchange Online
Exchange Online

A cloud-based service included in Microsoft 365, delivering scalable messaging and collaboration features with simplified management and automatic updates.

Answer accepted by question author
Ryan-N 15,740 Reputation points Microsoft External Staff Moderator
2026-08-12T18:50:42.7866667+00:00

Hi Alexsander,

Welcome to the Microsoft Q&A forum.

Thank you for sharing this information.

Please help me check the following steps. Based on the details you have provided, I would like to gather some additional information to better understand the issue:

  1. Please try using Outlook on the web (Outlook Web App) and verify whether the issue still occurs.
  2. Please try creating a new Outlook profile for the affected user and check whether the issue persists. To create a new Outlook profile, you can refer to the following documentation: Create an Outlook profile | Microsoft Support
  3. Please run the Microsoft diagnostic tool mentioned in the article below to check for Outlook connectivity issues:

Fix Outlook connection problems in Microsoft 365 - Exchange | Microsoft Learn

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Note: Running this diagnostic tool requires signing in with an account that has Microsoft 365 Administrator permissions in the tenant.

If you have any updates or additional information, please feel free to share them with me. I will continue to assist you with investigating and resolving the issue.

If the answer is helpful, please click "Yes" and kindly upvote it.

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  1. AI answer

    2026-08-12T18:38:09.9533333+00:00

    For a Microsoft 365 Group, the most relevant checks are the group delegation configuration and whether the issue is specific to Outlook client state versus the service.

    1. Verify the user has the correct group delegation permission.
      • For sending as the group, assign Send As to the user on the Microsoft 365 Group.
      • Group members can reply from their personal inbox using their own identity, but sending as the Group requires Send As or Send on behalf to be explicitly assigned.
    2. Confirm the permission on the group with Exchange Online PowerShell.
      • For Microsoft 365 Groups, use the group-specific approach to grant Send As:
         $groupAlias = "<GroupAlias>"
   $userAlias = "<UserAlias>"
   $groupsRecipientDetails = Get-Recipient -RecipientTypeDetails GroupMailbox -Identity $groupAlias
   Add-RecipientPermission -Identity $groupsRecipientDetails.Name -Trustee $userAlias -AccessRights SendAs
      • To verify whether Send As exists, run:
         Get-RecipientPermission -Identity "<groupname>" -Trustee "<user>"
      If no result is returned, the Send As permission is not present.
    3. Test in Outlook on the web.
      • Outlook on the web often reflects permission changes faster than the desktop client.
      • If sending works in Outlook on the web but fails in Outlook desktop, the issue is likely cached client data rather than the permission itself.
    4. Clear the cached group address in Outlook/OWA and reselect it.
      • Start a new email.
      • Type part of the group name.
      • When Outlook suggests the address, remove the cached suggestion.
      • Manually re-enter or reselect the group address from the address list.
    5. Test a new message from the group instead of replying to an existing thread.
      • If a new message works but replies fail, the problem can be related to the original conversation item rather than the send permission.
    6. Keep only the permission model that matches the expected behavior.
      • Send As: message appears to come directly from the group.
      • Send on behalf: message appears as the user on behalf of the group.

    Since removing and reassigning permissions was already tried, the next best isolation step is:

    • verify Send As is actually present on the Microsoft 365 Group,
    • test the same send action in Outlook on the web,
    • clear cached address entries and reselect the group address.

    If Outlook on the web also fails after the permission is confirmed, the available context does not provide a documented self-service fix for the specific error text "Illegal cross server connection. Cannot open mailbox."

    AI-generated content may be incorrect. Read our transparency notes for more information.

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