[eluser]Myles Wakeham[/eluser]
You might want to download PHPList and look at its code (its open source). It does exactly what you are asking for, and has features to control the rate of sending emails as my PHP Hosting sites will place limits on the max number of emails they will allow you to send per hour. Its not a CI solution, but it does what you are requesting and you could adapt its code to your needs.
Myles
