Whats a good service that allows our company to use its hundreds of thousands of subscriber emails?

Our Website is a vacation rental directory. People frequent our site to place inquiries for homes which owners showcase on our directory. When they inquire they opt into receive information from us. Does anybody have any recommendations for a list management system that can handle hundreds of thousands of email addresses without being incredibly strict on complienace. We've used some before but they tend to be for smaller businesses. What company works well for corporations or large companies like ours?

1) Charges start @ $5,000 US for consulting services. Thank you.

Questions About Email List Management Services

»A good list management tool for both email and snail mail?»Where do I find the 'safe sender list?'?»Whats a good service that allows our company to use its hundreds of thousands of subscriber emails?»How to contact Direct Energy management?»I got this email what shall i do?