Sign in to webmail click "Options" at the upper right corner, and select "Set Automatic Replies..." from the drop down list.
Under "Automatic Replies", the default option is "Don't send automatic replies" and the rest of screen is grayed out.
Click "Send automatic replies" option and then:
Determine whether you want the auto reply to be sent to people outside of the organisation.
A "Saving the Information..." box appears to indicate the process, once it disappears, click "My Mail" in the upper right corner to go back to Inbox. An auto reply will be sent to people who email you during the period you defined.
In Outlook 2010/2013/2016, click "File" tab, on the left, "Info" is selected by default (if not, click to select it), then click "Automatic Replies" button in the middle.
The "Automatic Replies" window pops up, by default "Do not send automatic replies" option is selected.
Click the radio button for "Send automatic replies" option.
When you're done with the message, if you click "OK" button, you'll get a warning message:
Click "Outside My Organization" tab:
Click "OK". An auto reply will be sent to people who email you during the time that you defined above.