PHP & MYSql programmers

I have a hosting server where many site are hosted for my clients. Yest its a shared hosting server working with Linux Platform. I am planning to provide a readymade emailing application to all my client from where they can send bulk email to the members registred with them. Any client who want to use the application can login with its particular login credentials to use it. They can create category wise mailing lists and then shoot out emailers to the list also provide a facility wherein they can upload their html mailers or jpegs of the mailers and mail it to list what i plan is to give a common entry point to all client via their own login credentials they come, login, create mailing lists and then shoot out mailers the from address should reflect their domains. all these clients are on the same server. Please get back to me if you need more clarification.