I restarted the mailing and it processed through all 510 emails
see attached screen shot.
There are any number of reasons why the server may hang when sending the emails. The server may have timed out the script, or it may have been a browser issue.
This is why the mailing tracks which emails have been sent, and if it hangs for some reason, allows you to restart the mailing without resending to addresses that have already ben sent to.