When you head out on vacation (you lucky dog, you), it's a good idea to set up an auto-response. This'll mean that every time you get an email, the server will automatically reply to the sender to let him know you're out of town. Trust me, unless you enjoy having to answer emails while you're sitting on the beach, you'll want to do this.
Another good use for auto-responses is when you're switching email addresses and don't want to have to send out a message to your entire contacts list to inform them all. Just configure the auto-response to say you're no longer receiving mail there, give 'em your new address, and you're golden.
But how do you set this up if you've got an @me.com, @mac.com, or @icloud.com email address? It's simple. First, go to iCloud.com, log in with your email address and password, and then click the giant Mail button.
Once you're in the Mail section, you'll see a gear icon in the upper-right of the window. Click on that and choose Preferences.
Now you're almost home free! Select the Vacation tab and toggle on Automatically reply to messages when they are received. Then just type in whatever you want your response to all incoming emails to be and press Done.
Easy peasy, right? Just don't forget to turn your auto-response off when you get back. In fact, it's a great idea to add a reminder or a calendar event so it doesn't slip your mind. I'm sad that iCloud doesn't allow us to set a date range of when the auto-response should be on like Gmail does. Get on that, Apple.