When you send email, you are sending data over and to private networks. The owners of the networks have the right to restrict what can and can't go through their network. In other words, you are a "guest" of their network. If you abuse your guest privileges, you can expect to be blocked from future access to their network. Getting your email blocked from a network is called blacklisting, and blacklists are maintained by most network operators.
Blacklisting services evolved out of the need for network owners to protect their networks from unsolicted bulk email, which can overwhelm a network and the company resources it serves. These services are third-party organizations that a network can pass email information through, to filter out known spammers before it arrives at the network. This allows network operators (in theory) to focus more on networking and their business at hand, and to delegate the blacklist administration and management to companies that are expert at finding and exposing spammers.
If you find your email blocked, we recommend using the blacklist monitoring site resources to find out how to work with the applicable blacklist operators to achieve an amicable solution. Remember that these services are trying to help protect company networks, and have a strong reputation for that reason. If you threaten, or otherwise make life difficult for the blacklist operator, you are very likely to worsen your situation. Even if you were wrongly accused, and your double-opt in mailing list is bullet-proof, work with the blacklist operators to achieve resolution - not against them.