How to fix "Webmail is turned off. You can turn it on by using the Server app on the server." on Lion Server?
According to Macgasm...


  There are still the same options, excluding the option for webmail, which has been removed. With the increase in use of iOS based devices, like the iPod touch, iPhone and iPad, all of which have the built-in Mail.app client, it’s possible that the use of web-based mail clients is rapidly decreasing. For me at least, the use of the web interfaces for my myriad of mail accounts is almost entirely non-existent. I cannot remember the last time that I logged into a web-based mail client to check my email. It only seems like a natural progression of the OS to remove a little-used feature. So, it’s not too surprising that Apple has removed any web-based mail client in OS X 10.8 Mountain Lion Server, likely preferring that users access mail services either via Mail.app on their iOS device or via Mail.app on their Mac.


Screenshot from Lion Server:



Screenshot from Mountain Lion Server: