Same thing for
123together.com. (They have rubbish Mac support, btw.)
I can access them through the web just fine, but Mail keeps asking for Exchange creds... the same ones that work via the web. Turns out that I also had to edit my Hosts file, to no avail. Not cool.
Why does OS X Mail have options for Internal and External Servers and Paths, where Entourage and Outlook don't? They just have "Domain," et al. That makes it a bit confusing/lame when dealing with the IT fellas.