If you send large volumes of email, we highly recommend Ongage. It has the most sophisticated tools we've found for managing emails, and maintains a platform which allows the use of multiple email service providers (important, if like us, you've been emailing for a long time and have seen many ESPs go out of business suddently).
We've been using them for years. Yes, we do get a commission if you end up using them, but would recommend them anyway. If you have questions about our experience with them, feel free to give us a ring at 201 871 0474, or give us your contact information below and we'll send you an information package.