Apple has effectively abandoned[1] the server room. All of the 'Enterprise' features of iOS require Microsoft Exchange[2]. For the plebeians, they make of point of supporting what ever is most popular: The email setup has custom paths for Gmail and AOL.

[1]: http://en.wikipedia.org/wiki/Xserve

[2]: http://manuals.info.apple.com/en_US/Enterprise_Deployment_Gu...