Blacklists are “third-party” data sources, informing receiving email servers whether the sender or the email content, should be considered “spammy”. As a “third-party”, they don’t actually block mail, but email servers may choose to accept or reject the recommendations from blacklists.
There are hundreds of blacklists, in every range of sophistication, from cutting-edge, enterprise-grade systems using artificial intelligence, to hobbyist blacklists run as personal projects. Some blacklists provide data on suspicious IP ranges, others list sending domains found in email headers, and still others list domain names found in email content, such as clickable links.
Spam filters often implement several blacklists simultaneously, to provide comprehensive data on all parts of an email, while ignoring other blacklists that are inaccurate, out-of-date, or idiosyncratic.
If you suspect that your mail is being rejected due to a blacklist, please contact SharpSpring’s Deliverability Support Team, who can investigate the evidence, provide best email practices to avoid blacklistings, and work with ISPs and blacklists for resolution.