PHP Mass emailing

[medafor]

I work for a small marketing company that has asked me to develop a PHP application to send emails for their company without getting blacklisted or emails showing up as spam. I have a fairly good understanding of PHP (OOP) but I do not understand how to build this program. They need approximately 500,000 to 1,000,000 emails sent per month. They are ready to buy ip addresses, SMTP servers etc but I need to code the program to work. So basically what I'm asking is what is the breakdown of the different parts of an application like this needs and how it can function without being blacklisted or considered spam. I apologize if i confused anyone on this topic, its just that i used the mail() function numerous times and many of the mail keeps going to spam. This is a legitimate pharmaceutical company as far as i know, they just contacted me and asked me if i could do this so thats why im here to see if it is possible or not. I know there are companies out there that can do this but they charge from $500 upwards and we cannot afford this. If there are specific equipment that needs to be bought we are ready to buy it to offset long term costs.Any help is appreciated, thank you!