Your mailbox
Select File > Automatic Replies.
In the Automatic Replies box, select Send automatic replies.
On the Inside My Organization tab, type the response that you want to send to teammates or colleagues while you are out of the office.
Note: Sending automatic replies to anyone outside my organization will send your automatic reply to every email, including newsletters, advertisements, and potentially, junk email. If you want to send automatic replies to those outside your organization, we recommend choosing My contacts only.
Other mailboxes
Login to OWA.
Select Outlook.
Click on your name or initials in the top right hand corner and select Open another mailbox.
You can then search for the person's mailbox that you need to connect to. Once opened, go to Settings.
You will see Automatic Replies underneath the Email section for you to be able to activate.