O365 – Select which mailbox to send email from Outlook Client (Multiple Mailboxes)

1. You can select From once you create a new message 

If you cannot, you need to setup by follows: 

1. Choose options -> … 

2. Choose From 

3. Select From -> other Email address (input your shared mailboxes) 

4. Then you can change the sender when sending an email from shared mailboxes.