Even if your lists are opt-in lists exclusively, you may fall victim to spam filtering and blocking if your email marketing service provider is also host to at least one email "marketer" that sends spam spam. If the large blackhole lists see spam coming from a provider and they don't act correspondingly, the provider will be blocked as a whole, including all the innocent marketers.
Make Sure Your Email Marketing ASP is Opt-In Only
That's why you should make sure
- your email marketing ASP is not on any blackhole lists and
- has a strict anti-spam policy and acts quickly when a spamming client shows up.