Out-of-office automatic email replies are useful to let people know that you aren't viewing or responding to your emails because you are on vacation or away for any other reason. Every email provider worth its salt has the option somewhere, including Outlook. If you use Outlook to manage other emails like your Gmail or iCloud accounts, you should go directly to the source to set up an out-of-office reply (i.e. log in to Gmail to set up automatic replies for that account).
Email and Messaging
The stock iOS Mail app does a fine job of bringing multiple email accounts into one place. Apple naturally integrates the Mail app across iOS for a variety of purposes, including smart Siri suggestions and Notification Center widgets. Just browsing your inbox is usually the easiest way to see which messages are a priority.
The out-of-office reply is a handy feature that lets you respond to emails automatically with a message saying that you are out of the office, or anything else you want it to say. Just about every email provider offers this feature, though Apple's Mail app does not have the option - you have to set it up on your Mac with Mail rules, and they are client-side, which means your Mac has to be on the whole time you are away in order to respond with your message.
The "out-of-office" reply is a commonly used automatic email reply that lets people know you will be away from your email for an amount of time, so they won't think you are ignoring them. Just about every email client has this tool, including one of the most popular - Gmail. Apple Mail and other email managers sometimes don't let you set up out-of-office responses. For Apple Mail you'd have to do it in a roundabout way, setting up rules on your desktop Mail app. It's a better idea to set it up at the source.
There are many apps that will manage your email accounts besides the iOS Mail app - Thunderbird, Outlook and Gmail will all do the job, to name a few. Outlook is popular with a lot of businesses, so if your work email is managed by Outlook you might want to add your other emails just to be able to access them all in one place. Or maybe you prefer the Outlook interface to that of Mail.