I am working on developing an HTML email newsletter system for a client. Previously I have developed similar systems, which used the PHP mail function to send the email. This works fine, however, with larger lists the script can take a very long time to complete its execution. This new email system, which I am creating, sends thousands of emails a week and each set of emails must be initiated by the owner. I have heard of people using courier to create mailing lists and then sending to those lists. Other than that I can’t think of an efficient way to create the system. If anyone knows of any articles, tutorials, or has created something similar any advice or direction would be greatly appreciated.