It's now YEARS past your original post. I have the same problem. Using iCloud from Safari, I cannot draft and send an email.
I stripped my machine down to a newly reformatted hard drive, and reinstalled the system. On a completely clean machine, I cannot send an email from the web version of iCloud. And this is a big deal, as I use my mail app for work exclusively - I don't want any cross traffic there. My job should not have access to personal emails like bank statements, doctors appointments, etc. The same is true of my iPhone - it's "work only" emails.
iCloud, if it functioned as it was intended to function, would allow me to check and respond to personal emails anytime, anywhere. As it is now, I can only do these things from home. As someone who works at a variety of client sites, and from my car, this is a big deal. Seeing that this has been an ongoing problem for years... Well, color me unimpressed. I have to pay an annual fee for my Apple ID; this functionality is a part of that package.
FIX IT ALREADY, APPLE...
The original post was in 2012. Now it's 2014.
The ability to send mail through iCloud is not a trivial matter.