I don't even know if this is possible...
Could I set up my mail server on my Rackspace cloud to use gmail but retain the email address email@example.com?
This question came from our site for professional and enthusiast programmers.
Probably. You would need to sign up for Google Apps for your domain. You would need to set up your DNS as the instructions will tell you so that Google becomes your mail exchanger. Then, you would need to get your mail server to use it. Instructions for postfix, for example, are here. You should be able to Google to find instructions for other mail servers, but that should get you started.