"Unable to verify account information" when setup on IPhone 4.

I have tripple checked my account information. The server is correct, along with the username and password.
I saw posts that suggested that some exchange server hosts may not allow IPhones. Other co-workers in my office have it setup on their IPhone so I know that isn't the case. The only thing that I can think of is maybe the optional domain space. However, I looked over my settings in my Microsoft Outlook and as far as I can see there isn't a domain listed. Unless I am overlooking it.

Any ideas?