That's it. We have now written the complete application. Certainly you can perform many enhancements to this system. Some suggestions are to:
Allow users to change names, email addresses, and/or passwords
Collect more information about the user
Allow other information to be inserted into the email message
Send encrypted emails
In the meantime, we've written an application that will allow a user to indicate certain interests when subscribing to the list, to selectively send email messages, and to insert personalized information into those messages. You'll want to enhance and bulletproof the application, but you should now have all of the tools you need in order to do so!