Maybe I'm a little confused here, but if it's webmail, which is tied into your site. Shouldn't your uname and password be your uname and pass on the site?

IF you go to hotmail and want to set up a pop 3 there so you can get your mail from another provider, you don't put in your uname and pass for outlook.

Have you set up your smtp correctly?

It appears that it is looking for a mailbox that should already exsist for your domain. Is your mail based on a single account?