Posted: Thu 28 May 2026, 11:30am.
We are aware that some users are currently experiencing an issue when sending emails from mailboxes they are a delegate of.
Users may receive the following error message when attempting to send:
"This message could not be sent. You do not have the permission to send the message on behalf of the specified user."
This occurs even though the correct Send As permissions are in place. We are actively investigating the issue and have raised it with Microsoft for further support.
Workaround
If you need to send emails from a delegated mailbox, you can use the web version of Outlook:
- Go to:
- Sign in with your own account
- Select your profile icon (top right corner)
- Click "Open another mailbox"
- Search for and select the mailbox you need access to
- Click Open
This will open the mailbox in a new browser tab, where you will be able to send emails successfully.
OR
You can open Outlook New (now solely named Outlook in the windows start menu) and follow the steps below to add the shared mailbox and send from there
Outlook New:
To add more functionality to shared mailboxes through Outlook new:
- In the top left of Outlook New select File and then Account Info
- Ensuring that you're on the Account tabs on the left hand side menu select the Shared with me option
- If you're missing the shared mailbox you can add it here using the + Add option
- Expand the shared mailbox using the downward arrow and then select Convert
- You will then see a message stating "To get full functionality out of your shared mailbox, restart Outlook". Select the Restart Outlook option.
- Use Outlook New as normal.
We will provide further updates as soon as more information becomes available. Thank you for your patience.
We have now implemented a fix which should start to resolve the issue preventing users from sending emails from delegated mailboxes.
If you continue to experience issues when using Outlook Classic, we recommend clearing the Outlook cache by following the guidance available here:
https://www.sussex.ac.uk/its/help/guide?id=306
As a reminder, the new version of Outlook (Outlook, formerly “Outlook New”) does not rely on a local cache, and using this version is therefore recommended where possible.
Outlook (New) – Shared Mailbox Setup
To ensure full functionality when accessing shared mailboxes in Outlook (New), please follow these steps:
- In the top left of Outlook, select File, then Account Info
- In the left-hand menu, ensure you are on the Accounts tab and select Shared with me
- If the shared mailbox is not listed, click + Add to include it
- Expand the shared mailbox using the dropdown arrow, then select Convert
- When prompted with “To get full functionality out of your shared mailbox, restart Outlook”, select Restart Outlook
- Once restarted, use Outlook as normal
We will continue to monitor the service. If you experience any further issues, please contact the IT Service Desk.
