Emails Not Recieved By Server From MessageLabs

We have many websites, each with their own domain and e-mail accounts hosted on a remote server. We just setup a second of these domains with messagelabs, the first of which is working fine. However, for some reason we arn't recieving any e-mails for this second domain, and apparently messagelabs can't connect to our server to send the messages. The ip they were using to send the email is correct, so I was just wondering if anyone here had any ideas? I assume it's a problem our end...we're running Microsoft Windows Server 2003 for Small Business Server (Service Pack 1) if that helps. I've been looking around the Microsoft Exchange System Manager, but to be honest I don't really know what I'm looking for or exactly where to look.

Any help would be appreciated!