Send on behalf

Hi @Rosie Brunson

Thank you for posting your question in Microsoft Q&A. 

To delegate your user to send as/ send on behalf, you could follow these steps to configure it (Please contact your administrator to complete the setup): 

1.Go to Admin Center-> click Users -> Active Users.  

In the list of mailboxes, select the mailbox that you want to assign permissions for. 

2.Click Mail -> Mailbox Permission and configure one or more of the following permissions Send As/ Send on behalf/Read and manage (Full Access) 

3.Select Edit under the appropriate permission, and then click Add members. 

4.Select the user or group from the list.  

You can also search for users or groups in the search box by typing all or part of the name, and then clicking  Search. 

5.When you're finished selecting delegates, click Save > Confirm. 

You can refer more via:  

Manage permissions for recipients in Exchange Online | Microsoft Learn 

If you've already configured the permissions but the user is still unable to use send as/ send on behalf via Outlook, this may be due to outdated cache or address book information. Outlook might display an error such as: "You do not have permission to send the message on behalf of the specified user" even after permissions have been correctly granted.

To resolve this issue, you can try the following steps:

• Clear Outlook cache to refresh permission and address book data.
• Use Outlook Web App (OWA) to verify if the issue persists there. OWA often reflects permission changes more quickly than the desktop app.

If you have any additional concern, feel free to comment below. I be more than happy to assist.

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