When you try to draft a reply to an email in a shared mailbox using Copilot in Outlook, the Draft with Copilot button may be grayed out, missing, or produce an error message instead of generating a response. This problem occurs because Copilot requires specific permissions and licensing conditions that shared mailboxes do not automatically inherit from the user. This article explains the root cause of this limitation and provides the exact configuration steps to resolve it.
Key Takeaways: Fix Copilot Reply Drafting in Shared Mailboxes
- Microsoft 365 admin center > Exchange admin centers > Shared mailboxes > Mailbox delegation: Grant Send As or Send on Behalf permission to the user.
- Outlook on the web > Mailbox settings > Copilot in Outlook: Enable the Copilot toggle for the shared mailbox after permissions are set.
- Microsoft 365 admin center > Settings > Org settings > Copilot: Verify that Copilot is enabled for the tenant and licensed users.
Why Copilot Cannot Draft Replies in Shared Mailboxes
Copilot in Outlook uses the email context of the current mailbox to generate responses. For shared mailboxes, Copilot cannot read the mailbox content, identify the conversation thread, or access the sender’s intent unless the user has explicit delegate permissions. Without Send As or Send on Behalf permissions, Copilot treats the shared mailbox as a read-only folder and disables the Draft with Copilot feature. Additionally, the user must have a valid Copilot for Microsoft 365 license assigned. Shared mailboxes themselves do not require a license, but the user accessing them must be licensed and the Copilot service must be enabled in the tenant.
Permission Requirements
The user must have at least Send on Behalf permission on the shared mailbox. Full Access permission alone is not enough. Without this delegation, Copilot cannot verify that the user is authorized to send replies from that mailbox. The permission assignment must be done in the Exchange admin center or via PowerShell.
Copilot Service Enablement
The tenant administrator must enable Copilot for Outlook in the Microsoft 365 admin center. If the Copilot service is disabled for the organization, all users lose the Draft with Copilot feature regardless of mailbox type. This setting is located under Org settings > Copilot.
Steps to Enable Copilot Reply Drafting in a Shared Mailbox
Follow these steps in order. Each step must complete successfully before proceeding to the next.
- Assign Send on Behalf permission to the user
Go to the Microsoft 365 admin center at admin.microsoft.com. Select Exchange admin center > Recipients > Mailboxes. Find the shared mailbox and open its properties. Under Mailbox delegation, add the user with Send on Behalf permission. Click Save. Wait 15 minutes for the change to replicate across Microsoft 365. - Verify the user has a Copilot for Microsoft 365 license
In the Microsoft 365 admin center, go to Users > Active users. Select the user and open the Licenses and apps tab. Confirm that Copilot for Microsoft 365 is assigned. If not, assign it and wait 30 minutes. - Enable Copilot in Outlook for the shared mailbox
Open Outlook on the web. Right-click the shared mailbox name in the folder pane and select Shared mailbox settings. Expand Copilot in Outlook and turn the toggle to On. If this toggle is missing, return to step 1 and verify permissions. - Restart Outlook and test drafting
Close Outlook completely. Reopen it and select the shared mailbox. Open an email and click Reply. The Draft with Copilot button should now be active. Click it and enter a prompt to generate a reply.
If Copilot Still Has Issues After the Main Fix
If the Draft with Copilot button remains disabled after following the steps above, check these additional factors.
Copilot Tenant Setting Is Disabled
An administrator may have turned off Copilot for the entire organization. Go to Microsoft 365 admin center > Settings > Org settings > Copilot. Ensure the toggle under Allow Copilot for Outlook is set to On. If you are not an admin, contact your IT department to verify this setting.
Outdated Outlook Client Version
Copilot in Outlook requires a minimum build version. For Outlook on the web, no update is needed. For the Outlook desktop app, ensure you are using Microsoft 365 Apps version 2402 or later. Go to File > Office Account > About Outlook to check the version. If outdated, run Microsoft 365 Apps update from File > Account > Update Options.
Shared Mailbox Not Added as a Separate Account
In the Outlook desktop app, the shared mailbox must be added as a separate account in the profile, not just as an auto-mapped folder. Go to File > Account Settings > Account Settings. Select your primary email account and click Change > More Settings > Advanced. Under Open these additional mailboxes, add the shared mailbox name. Restart Outlook.
Copilot Pro vs Copilot for Microsoft 365: Key Differences for Shared Mailboxes
| Item | Copilot Pro | Copilot for Microsoft 365 |
|---|---|---|
| License required for shared mailbox use | Not supported for shared mailboxes | Required on the user account |
| Outlook integration | Basic Copilot features only | Full Draft with Copilot in shared mailboxes |
| Permission dependency | None | Send on Behalf or Send As required |
| Admin control for tenant | Not available | Copilot toggle in Org settings |
Copilot Pro does not include the ability to draft replies from shared mailboxes. Only Copilot for Microsoft 365 unlocks this feature. If your organization uses Copilot Pro, you must upgrade licenses to enable shared mailbox reply drafting.
You can now enable Copilot to draft replies from any shared mailbox by assigning Send on Behalf permission, verifying the Copilot license, and enabling the Copilot toggle in Outlook on the web. After these steps, test the feature with a single email before rolling out to additional users. For advanced management, use the Exchange Online PowerShell cmdlet Set-Mailbox -GrantSendOnBehalfTo to assign permissions in bulk.