I'm working on a project that will basically be a Constant Contact like system. Mass emails that someone creates and we are the service which sends, opt-in, opt-out. Where I need help is planning for scalability. A single server emailing is easy but I want to plan for 1 million emails a day and it could be 10,000 per hour. My thought is that I would need to setup an email cluster with some sort of load balancing deciding which server will send out a batch of emails if another is processing a different users request. Bandwidth is not an issue. Am I correct in thinking an email cluster? I've never setup ISPC with multiple emails servers. My plan was a Perfect ISPC Wheezy setup with Courier....can Courier scale? I've made plenty of mistakes having to redo implementations because I didn't ask the experts ahead of time. Any advise or guidance would be appreciated.