O365 - Set a Vacation/Auto reply Message
Documentation Source: Microsoft.com
This topic shows you how to automatic replies in Outlook Web App using the Automatic Replies tab. Use automatic replies whenever you want to let people who send you email know that you won’t be responding to their messages right away. After automatic replies are turned on, they’ll be sent once to each sender. If you have a Microsoft Exchange-based email account, for example, a Microsoft Office 365 account, and you use Outlook and Outlook Web App, you can use either one to manage your automatic replies.
Set up an automatic reply
To turn on or modify automatic replies, go to the gear icon, then Options > Organize email >Automatic Replies.