Some email servers and SPAM filters reference "Blackhole lists," which are lists of IP addresses of known spammers, or "spam friendly" servers. If an IP address is on the list, they won't let your email through.
Anybody can setup a "blacklist" for their own email server. And they can post that list for the public to see and use. Inevitably, there are some irresponsible blacklist owners who blacklist servers out of spite. So, not all blacklists are created equal. Smaller blacklists, including SORBS, blackholes.five-ten and Spam Cannibal have less influence with the more popular, bigger ISPs. Most large ISPs only use the most reputable blacklists. This includes SpamCop, Spamhaus and URIBL.
How Blacklists WorkIf a server gets on a blacklist such as SpamCop, any email from the server is blocked by ISPs that use the SpamCop blacklist. SpamCop (and other reputable blacklists) usually have a threshold, and usually delist servers after a reasonable period of time.
It's rare, but sometimes a MailChimp customer will send an email campaign to a list of people that forgot they ever opted in (such as to a stale customer list). If a large percentage of those people report the email as spam, the MailChimp server will get temporarily blacklisted. One bad apple can spoil the whole bunch, so you can understand why we sometimes need to be "strict" in our terms of use.
Sometimes, Blackhole lists and SPAM filters don't just ban a specific server's IP address---they ban an entire BLOCK of IP addresses. This basically blocks all email from all the servers setup on that general IP range (for instance, 69.20.10.???). This is a drastic measure, but SPAM is a drastic problem. If your IP address is on a blackhole list, it might not actually be because of you. It might be because someone in your IP range is a spammer.
A good place to see if an IP address is on a blackhole list is at: http://multirbl.valli.org/lookup/