I can't speak much for Constant Contact's control panels and what not. But what I can tell you is that they're on a mailing list that a lot of postmasters are

I can't speak much for Constant Contact's control panels and what not. But what I can tell you is that they're on a mailing list that a lot of postmasters are on, and whenever someone has a problem with one of their clients[1], they're very responsive, usually within a matter of hours (sometimes minutes). And that has, without a doubt, kept them in good graces. So my point here is twofold: A. Constant Contact is, at least, a healthy player in the field, and you're likely to get good throughput with them. B. Don't try to pull any fast ones over these companies, if they don't notice themselves, they'll hear about it very quickly. Make sure you keep your lists clean and you're following every single guideline they give you (and they're usually happy to help you). [1]. Which happens to any service at some point; they can't exactly hand-vet every mailing that goes through their service.