Use automatic replies to tell people you won't be responding right away to their email messages. Automatic replies are sent once to each sender.
Instructions for Outlook Web App are below.
Outlook Desktop App:
-
Select File > Automatic Replies (Out of Office).
-
In the Automatic Replies box, select Send automatic replies.
Optionally, check Only send during this time range. This will turn off automatic replies at the date and time you enter for the end time. Otherwise, you'll need to turn off automatic replies manually.
-
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 is off by default. To turn it on, select the tab and check Auto-reply to people outside my organization. Be aware: this 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.
-
Select OK to save your settings.
Turn off automatic out-of-office replies
When Outlook is setup to send automatic replies, you'll see a message under the ribbon with this information. Select Turn off to disable automatic out-of-office replies. If you want to modify the dates for your automatic reply or the message sent, use the steps above to modify your settings.
Outlook Web App:
-
At the top of the page, select Settings
> View all Outlook settings > Email > Automatic replies. -
Select the Turn on automatic replies toggle.
-
Select the Send replies only during a time period check box, and then enter a start and end time. If you don't set a time period, your automatic reply remains on until you turn it off manually
-
In the text box that appears, type a message to send to people during the time you're away.Note: Send replies outside your organization is off by default. To turn it on, check Send replies outside your organization. Be aware: this 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 checking Send replies only to contacts.
-
When you're done, select Save at the bottom of the window.
Turn off automatic out-of-office replies
If you didn't set a time period for automatic replies, you'll need to turn them off manually. To turn off automatic replies, sign in to Outlook on the web, choose Settings 
> View full settings > Email > Automatic replies and then select the Automatic replies on toggle. Select Save at the bottom of the window.