Add Send on Behalf Permission in Exchange Online

In Exchange Online (Microsoft 365), you can allow a user to send on behalf of another user’s mailbox, shared mailbox, or distribution list.

In Exchange, you can provide one of three types of mailbox privileges:

• Send As — allow the delegate to send e-mails from the mailbox;
• Send on Behalf — allows the delegate to send messages on behalf of another user. In this case, the real sender of the email will appear in the From: field. This is a way for the recipient to know that someone else is replying to their e-mail;
• Full Access — allows you to view, create, and delete items in the other user’s mailbox.

You can grant permissions to send on behalf of a mailbox through the Exchange Admin Center or by using PowerShell:

1. Sign in https://admin.exchange.microsoft.com/#/;
2. Navigate to Recipients > Mailboxes;
3. Open the properties of the mailbox you want to assign permissions to;
4. Go to Delegation and click the Edit button under Send on behalf (or click the Mailbox delegation button in the actions list);
5. Click Add Manager and select the user you want to delegate permissions to from the list;

A message should appear:

Mailbox permissions were added to these mailboxes
Selected users have been added successfully. The changes are saved and will appear‎ within 5 minutes.

You can also grant Send on Behalf permissions using PowerShell:

1. Connect to the Microsoft 365 tenant using the Exchange Online PowerShell V3 (EXO V3) module:
   Connect-ExchangeOnline
2. The Set-Mailbox cmdlet is used to grant the right to send on behalf. For example, to allow the user test_jackson@theitbros.com to send on behalf of the mailbox salary@theitbros.com, run the command:
   Set-mailbox salary@theitbros.com -Grantsendonbehalfto @{add=”test_jackson@theitbros.com”} 
3. Check that you have successfully added the Send on behalf permissions to the user:
   Get-Mailbox salary | ft Name,Grantsendonbehalfto –wrap
4. The command returns the user’s GUID in the GrantSendOnBehalfTo column. To get the mailbox name from its GUID, use the Get-Mailbox cmdlet:
   Get-Mailbox xxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxx 

Now send an email on behalf of another mailbox:

1. Run Outlook or open your mailbox with Outlook for web;
2. Create a new e-mail;
3. Go to the Options tab and check the box Show from;
4. Click From > Other email address > and select the mailbox in the address book on behalf of which you want to send an email;
   
5. Fill in the recipient, subject and body of the email. Then send it;
6. In the mailbox of the recipient, your account will be displayed as the sender who has sent the e-mail on behalf of another mailbox.
   

An error may occur in Outlook when you send an e-mail on behalf of someone:

You can’t send a message on behalf of this user unless you have permission to do so.

In this case, check that you have correctly assigned permissions. In some cases, you may need to wait a while for replication in the Azure tenant.

If you want to revoke the Send on Behalf permissions for a mailbox, run the command:

Set-mailbox salary@theitbros.com -Grantsendonbehalfto @{Remove="test_jackson@theitbros.com"}

Use the following PowerShell one-liner to list all mailboxes in the Exchange Online tenant that have been assigned Send on behalf permissions:

Get-Mailbox -Filter {GrantSendOnBehalfTo -ne $null} | select GrantSendOnBehalfTo,PrimarySmtpAddress,RecipientTypeDetails

Note. You can also use PowerShell to manage Exchange mailbox calendar permissions.