How to Configure Copilot in Outlook for Send-On-Behalf-Of Permissions

When you manage another person’s mailbox in Outlook, you may need Copilot to draft emails that appear to come from that person. Without proper configuration, Copilot will use your own identity and signature instead of the shared mailbox owner’s. This happens because Copilot in Outlook respects the same Send-On-Behalf-Of permissions that Exchange Online enforces for delegated mailboxes. This article explains how to grant the correct permissions and configure Copilot so it drafts messages using the delegate’s identity.

Overview of Copilot Send-On-Behalf-Of Permissions

Copilot in Outlook can draft emails on behalf of another user only when the correct Exchange Online permissions are in place. The feature relies on the same Send-On-Behalf-Of permission that allows one user to send emails that appear to come from another user. Without this permission, Copilot will always use the primary mailbox of the signed-in user.

The permission is set at the mailbox level in Exchange Online. It applies to both manual sending and Copilot-generated drafts. When Copilot drafts a message, it checks the sender identity at the moment the draft is created. If the delegate mailbox is not selected, Copilot uses the default account.

Prerequisites for this configuration include:

• An Exchange Online license assigned to both the delegate and the mailbox owner.
• Global admin or Exchange admin role in the Microsoft 365 admin center.
• Copilot for Microsoft 365 license assigned to the delegate user.
• Outlook for Windows version 2302 or later, or Outlook on the web.

Steps to Grant Send-On-Behalf-Of Permission in Exchange Admin Center

These steps are performed by an administrator in the Exchange admin center. The permission applies to the mailbox that Copilot will draft messages for.

1. Open Exchange admin center
   Sign in to admin.microsoft.com and go to Admin centers > Exchange. Or go directly to admin.exchange.microsoft.com.
2. Navigate to recipient mailboxes
   In the left navigation, select Recipients > Mailboxes.
3. Select the delegate mailbox
   Click the mailbox that you want Copilot to send messages on behalf of. This is the mailbox of the person whose identity you want to use.
4. Open mailbox delegation settings
   In the mailbox properties pane, select Mailbox delegation.
5. Add Send On Behalf permission
   Under Send on behalf, click Edit. In the dialog, search for the user who will use Copilot. Select the user and click Add. Click OK.
6. Save changes
   Click Save at the bottom of the mailbox delegation pane. The permission takes effect within 30 minutes.

Steps to Configure Copilot to Use the Delegate Identity

After the permission is granted, the delegate user must configure Outlook to use the shared mailbox identity when drafting with Copilot.

1. Open Outlook and select the shared mailbox
   In Outlook for Windows, go to File > Account Settings > Account Settings. Select the shared mailbox account and click Change. Ensure the mailbox is added as an additional mailbox. In Outlook on the web, click your profile picture and select Open another mailbox. Enter the delegate mailbox address.
2. Open Copilot pane
   In a new email message, click the Copilot icon in the ribbon or press Alt+I. The Copilot pane opens on the right side.
3. Select the delegate identity
   In the Copilot pane, click the Settings gear icon. Under Account, choose the delegate mailbox from the dropdown list. This tells Copilot which identity to use for drafting.
4. Draft a message with Copilot
   Type your prompt in the Copilot pane. For example: “Draft a thank-you email to the client about the project update.” Copilot will generate the message using the delegate’s signature and identity.
5. Verify the sender identity
   Before sending, check the From field in the email. It should show the delegate mailbox address. If it shows your own address, click the From button and select the delegate mailbox manually.

Common Issues with Copilot Send-On-Behalf-Of

Copilot still uses my identity after granting permission

This occurs when the delegate mailbox is not selected in the Copilot pane settings. Open the Copilot pane, click Settings, and verify that the correct mailbox is selected under Account. Also confirm that the shared mailbox is added to your Outlook profile.

Copilot cannot access the delegate mailbox at all

If Copilot shows an error that it cannot access the mailbox, the Send-On-Behalf-Of permission may not have propagated yet. Wait 30 minutes and try again. If the error persists, have an admin verify the permission in Exchange admin center under Mailbox delegation.

The From field shows my address even after selecting the delegate identity

Copilot sets the sender identity only for the draft content. The From field is controlled by Outlook’s own sender selection. Click the From button in the email header and choose the delegate mailbox address. This is a separate setting from the Copilot identity.

Copilot Send-On-Behalf-Of vs Full Access Permission

Item	Send-On-Behalf-Of	Full Access
Purpose	Send emails that appear to come from the delegate	Read, manage, and send from the delegate mailbox
Copilot identity	Uses delegate identity when selected in Copilot pane	Uses delegate identity only if mailbox is added as shared
Permission required	Send-On-Behalf-Of in Exchange admin center	Full Access in Exchange admin center
Sender display	Shows delegate name with “on behalf of” tag	Shows delegate name without tag
Copilot draft behavior	Uses delegate signature and identity	Uses delegate signature only if mailbox is primary

Configuring Copilot for Send-On-Behalf-Of permissions requires two steps: setting the Exchange permission and selecting the delegate identity in the Copilot pane. After you complete both steps, Copilot will draft messages that correctly represent the mailbox owner. For advanced scenarios, test with a simple prompt first to confirm the sender identity appears as expected. If you manage multiple delegate mailboxes, create a separate Outlook profile for each to avoid identity conflicts.