Your first stop should be "Queues" under your server computer object in the Exchange System Manager. There should be a queue containing your outbound email in "Retry" state. Highlighting that queue should show you, under the "Additional queue information" frame, some reason why the queue is in "Retry" state.
You should definitely try to TELNET to the smart host's name, port 25, (
telnet smart-host.example.com 25) from the server computer to see that you get an SMTP server banner. If you can't get that then message delivery isn't going to work.
Assuming that your server is able to reach the smart host's SMTP server you can use SMTP protocol logging to troubleshoot why your smart host isn't taking messages anymore. Personally, I'd sniff the traffic either with Wireshark or the built-in Network Monitor, rather than doing SMTP protocol logging, but that's just personal preference.
You will have success if you work to understand how the process should work and understand the protocol you'll have a much better time troubleshooting this.