Hi Paul,
I'd check to make sure you don't have any other campaigns going on that could bog down your server. If all the test emails went to people on the same company server, I'd try a live test to a gmail or yahoo address as well. It could be an issue with the internal server firewall blocking the emails. If there are no other campaigns going on and the live test doesn't work, I'd contact support. There may be a backlog issue on their end.