I think retunpath currently is the leader in deliverability but they could use some work in the customer service department. Read---if you do not know what you are doing then do not get a returnpath account. Their email preview tools are excellent and their reporting tools are beyond industry standard. Their spam filter tester also works marvels. My advice is if you go with this company make sure you get a rock star account manager who will go the extra mile to help you out.