How to setup Auto-Reply and Forwarding in Rackspace Mail

To setup Auto-reply, login to webmail at https://email.rackspace.com/ and click on "Settings" (top right corner beside your email)
In "Settings" window, click "Incoming email", setup auto-reply: 

and save

You can find more details here: https://support.rackspace.com/how-to/rackspace-webmail/