1. In Outlook Web App, click on the Options icon (gear icon).

2. Select Set automatic replies. 

3. You're brought to the automatic replies page. Select Send automatic replies. 

4. If you would like the replies to be sent for a limited period of time, set the dates in the Start time and End time fields. 

5. Now edit the email template for the replies to your colleagues under the Send a reply once to each sender inside my organization with the following message section. 

6. You can choose to activate the out-of-office automatic replies for your external contacts too. Check the Send automatic reply messages to senders outside my organization.

If you want to send the replies only to people in your contacts list, select Send replies only to senders in my Contacts only. Otherwise, let the default option selected (Send replies to all external senders).

7. Make sure you click on save before leaving the page. A warning will remind you to do so if you don't when you click on another page link.