Office 365 (Outlook on the web) - Automatic replies (formerly Out of office assistant)
This topic shows you how to use 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.
Manage Automatic Replies
To turn on or modify automatic replies, review Microsoft instructions.