Blacklist

It is common for an ISP to a use a blacklist to determine which e-mails should be blocked. Blacklists contain lists of domains or IP addresses of known and suspected spammers. Unfortunately, these blacklists also contain many legitimate e-mail service providers. Just a few spam complaints can land an e-mail service provider or IP address on a blacklist despite the fact that the ratio of complaints to volume of e-mail sent is extremely low.
Marketing Strategy: Spam-Free E-mail Marketing Campaigns