There are two types of lists that e-mail marketers are...or need to be...well aware of. First, there's the list you don't want to be on. It's the black list. ISP's (Internet service providers) use different methods and techniques to deliver e-mails that mailing list people want, and to block e-mails that people don't want. One of the primary techniques used is the reputation of the IP address of the sender, linked with the reputation of the ESP (e-mail service provider).
However, having an unknown...or little-known IP address...can be enough to keep your e-mail from getting through. This can happen even if your ESP is well-known and has a blue ribbon reputation. And...if you decide to switch your e-mail mailing list service provider, your combined IP/ESP rating will begin again in order to re-establish your ranking with the ISP. Of course, the ISP will also always check their databases to see if the email sender's IP address is on the list of known "spammers."
Your IP address can erroneously get on that list, or through various errors you've made when sending e-mails. If this somehow seems too confusing or unfair, take a moment to remember how we got here. The proliferation of spam, or mailing list undesired e-mail, some of them with serious viruses, were responsible for spam cops and other "cyber supers" to "protect" us.