So what is Autodiscover?
Autodiscover is a service that allows users to easily configure their email client knowing only their email address and password. E-mail clients that make use of Autodiscover include Outlook 2007, Outlook 2010, Outlook 2011, Mac Mail, Entourage 2008 WSE and many smartphones. Autodiscover also enables additional features for Microsoft Exchange mailboxes such as: Downloading the Offline Address Book, viewing Free/Busy time in calendar, and Out-of-Office assistant.
Note: Autodiscover for Mailtrust and Microsoft Exchange Email configures email clients for IMAP and SMTP, however, it is not compatible with Outlook 2011 and Mac Mail.
How do I enable Autodiscover for Rackspace Mail?
Contact your DNS hosting provider and ask them to create the following CNAME record. If you have access to the control panel for your DNS, you can make this change yourself.
Record Type: CNAME
TTL (Time To Live): lowest possible
Note: Some DNS providers require your full domain name in the hostname field; e.g. if your domain is example.com, your hostname would be autodiscover.example.com