Search your email soul, if you will. Unlike an acquisition campaign that can yield click-throughs and conversion data within hours or days, retention email is about the long term. Whether you're building awareness of your company as a thought leader or educating your target audience about product benefits, it takes time. That said, here are the categories providers fall into. List hosting, delivery, and management companies include SparkLIST, L-Soft, and Lyris.
Pricing is based on volume of messages delivered.
A few more bells and whistles. You can segment your house file and personalize messages based on your own business rules, as well as determine timing and frequency of delivery. One well-regarded product in this category is MessageMedia's UnityMail Express. Other midrange ASP vendors are EmailFactory and CheetahMail. And there are many others.