OK first of all I would not take this part of the requirement on yourself. It will always be much more effort to build your own email infrastructure and manage the deliverability and ISP relations than it will be to integrate with an ESP (Email Service Provider).
By far your easiest and most effective route is to look for an email marketing company that suits your requirements that has an API you can hook into to execute the send. Most reasonable providers have this functionality.
By using one of these ESPs you benefit from their deliverability, isp relations and it will provide you with many more tools like the ability to target specific customers based on their preferences or looking at how your email will look in each mail client e.g outlook 2010 will display the email differently to gmail. All this you will never get developing the solution yourself.
ESP's usually charge a small monthly fee for access to their API's and to have an account open and then a charge per 1k emails sent on top. Far cheaper than you spending many hours or days developing a solution yourself.
Hope this helps,
Charlie
