Thanks, vlad! That helps a lot! One suggestion that might be helpful, though:
If you have any other applications that might be sending mail as the same user that are not using the "queue" code (another domain, etc.), you might want to make the limit something less than the 200 mails in the example (since that "200" represents your entire hourly quota), to allow for the other mail to be processed in addition to the queued mail and still remain under quota.