Our provider is Qwest but we access through MSN.
This is probably your key, your Qwest account information probably isn’t set up correctly with MSN (Hotmail is their email service I believe, is that what you’re using?), you can send because you’re sending through the MSN servers instead.
Try setting up your account separately with an email client and make sure everything is working there (your account login for instance)... here are some instructions from Qwest. Their tech support can probably help you with this part if you run into trouble.
Once you have that working and know the information is correct, then try to set it up with MSN again. They’ve got a couple different flavors of hotmail out there right now. But these instructions might help if you’re using the “Windows Live” version.
edit: and Microsoft has some additional info here regarding pulling together your email addresses.