Xuheki is the mythological god of electronic mail. Oh wait, the Wikipedia page is missing. Please do something about it. ;-)
As I've written all over this website, Xuheki is an email client. And (somewhat) server. But it only supports IMAP (and still it's not an IMAP server). Confusing, right?
Suffice to say that you can read and write mail with it, it works in a browser and you can access it from any computer (not just your computer) as long as you're using a decent browser.
Xuheki has a bunch of nice features, but keep in mind it's only version 0.1. It probably has a bunch of bugs too (but I hope you'll tell me about it). All in all it's a fine Webmail and I dare you to install it and throw some feedback my way. ;-)
