I just set up a new SBS 2011 server and am having trouble getting external email to send. I have found that the messages are stuck in the mail queue with a status of : "Retry"
The Last Error is the same for all which is: "451 4.4.0 primary target ip address responded with 421 4.2.1 unable to connect"
My ISP blocks port 25 so I changed the Send Connector port from 25 to 2525
I also added DNS forwarders of: 18.104.22.168, and 22.214.171.124 (Google Public DNS)