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How to Fix Shared Mailbox Send As Permission Errors in Outlook
Question details
Users are unable to compose new messages from a shared mailbox address in Outlook, despite having Full Access and delegation permissions configured.
- Product
- Microsoft Outlook
- Device & OS
- not provided
- Scenario
- Creating and sending a new email using the email address of a shared mailbox.
- Observed behavior
- Outlook returns an error message stating that the user does not have permission to send on behalf of the specified user.
Ensure you have access to the Microsoft 365 admin center or have a way to contact your IT administrator, as resolving this issue requires checking Exchange permission settings.
Verify and Re-assign Send As Permissions in Microsoft 365
The most common cause is missing or un-synced 'Send As' permissions. An admin must ensure the specific permission is actively granted.
Even if a user has 'Full Access' to a shared mailbox, this does not automatically grant the ability to send emails as that mailbox. The 'Send As' permission must be explicitly assigned and allowed time to replicate across the Exchange servers.
Navigate to the Microsoft 365 admin center and log in with your administrator credentials.
Expand the 'Teams & groups' menu on the left sidebar and select 'Shared mailboxes'.
Click on the affected shared mailbox. In the details pane, scroll down to 'Send as permissions' and click 'Edit'.
Click 'Add members', select the users who need to send emails from this address, and click 'Save'.
Test Mailbox Access in Outlook on the Web (OWA)
Testing in OWA helps determine if the issue is a local Outlook desktop client problem or a server-side permission issue.
Update the Offline Address Book in Outlook Desktop
If permissions work in OWA but fail in the desktop app, forcing a sync of the Offline Address Book can resolve local cache issues.
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Frequently Asked Questions
Why can I reply to emails in the shared mailbox but not compose new ones?
Replying often works if you only have 'Full Access' permissions. Composing a brand new email from the shared mailbox's address strictly requires 'Send As' or 'Send on Behalf' permissions to be explicitly granted by an administrator.
How long does it take for Send As permissions to take effect in Microsoft 365?
While changes in the Microsoft 365 admin center apply quickly on the backend, it can take anywhere from 15 minutes to 24 hours for the new permissions to fully replicate across Exchange Online and sync down to your local Outlook desktop client.
What is the difference between 'Send As' and 'Send on Behalf'?
'Send As' allows you to send emails that appear to come directly from the shared mailbox with no mention of your personal account. 'Send on Behalf' indicates to the recipient that the email was sent by you on behalf of the shared mailbox (e.g., 'User A on behalf of Shared Mailbox').
Will automapping automatically configure Send As permissions?
No. Automapping automatically displays the shared mailbox in your Outlook folder pane when you are granted Full Access. It does not automatically grant 'Send As' permissions, which must be added separately by an administrator.
