Exchange 2007 Not Sending External Emails

I have a newly-built Exchange Server 2007. I’ve set up everything I can think of, including the Send Connector which I know is a new step for this version. However, when I send an email from my Outlook 2007 client, I can see it sitting in the queue via the Queue Viewer, but it never goes out. It just sits in Retry.

As I said, the send connector has been configured, I’ve also tested DNS resolution using nslookup from the mail server, and that seems to work just fine as well. Any ideas?