Google is currently rolling out an update to Gmail to enable IMAP support on hosted accounts. IMAP is short for Internet Message Access Protocol and allows you to access your Gmail emails from other clients, like Outlook or Thunderbird.
The benefit of IMAP is that you’re able to check your mail directly on your mail server through a desktop mail client or mobile mail client, as opposed to downloading the messages to your computer.
To check and see whether IMAP’s been enabled on your account, go to http://www.google.com/a/ and enter your domain name. Then log into your email account. Once logged in, click Settings (up top right) and see if you have a tab named Forwarding and POP/IMAP. If you don’t have it yet, don’t worry… you’ll see the feature on your account in the coming days.
For more information on IMAP and Gmail, Google explains it in greater detail in this help entry.