To setup Automatic replies follow these steps:

From Outlook on the web:

Click the gear icon for settings in the upper right-hand corner 

In Mail go down and select Automatic Replies:

From here customize appropriately based on need as well as if there needs to be a message for senders outside of the organization.

From the Outlook desktop application:

Click the gear icon for settings in the upper right-hand corner 

Under Accounts select Automatic Replies



From here customize appropriately based on need as well as if there needs to be a message for senders outside of the organization.