Spam traps and the occurrence of blacklists are two indicators of mailing to contacts who are no longer engaged or contacts who may not even be real people. On ESPs where one pays per contact held in the database or per emails sent annually, the mailing to spam traps can be as costly as they are dangerous - hitting too many could result in up to 80% of mail being blocked. Blacklists are the other side of the coin where they jeopardize successful delivery to those real engaging contacts.
What they are and how they're triggered
Blacklists go hand in hand with spam traps in the sense that blacklists occur when a blacklist operated spam trap gets mailed to repeatedly by the same domain or IP address within a time frame. The result is that those IPs/domains are put on a blacklist that particular ISPs reference when letting mail through to their receivers. If the IPs/domains are on the blacklist, the ISPs will bounce or block mail from getting through. If they are not on the blacklist in question, they will let mail through and it will be subject to the usual reputation filters and such.
Blacklists of note
There are many blacklists out there and many of them are run by anti-spam activists. Since no blacklist can affect outgoing mail without the ISPs complying, there are only a few worth noting. They are, Spamhaus, SpamCop, Invaluement, and SORBS. Of these blacklists, three out of the four affect a noticeable amount of mail, with the exception of SORBS. Spamhaus affects the largest percentage of mail hovering around 80% at most ISPs, notably Verizon Media Group (Yahoo, AOL, Verizon) and Outlook (Outlook, Hotmail, Live, MSN).
Blacklist remediation process
Tripping a blacklist is indicative of reaching out to a large and at times, chronically unengaged audience group. Despite the impact of the blacklist on the percentage of mail being delivered, hitting a blacklist is a litmus test of how far back in the database is too far. The further back in engagement (30 days engaged vs 365 days engaged) is mailed, the higher the risk of mailing to a spam trap owned by blacklist operators. The more consistently that trap is mailed to, the higher the potential of having the sending IPs placed on the blacklist.
To course correct, the best thing to do is to isolate the campaign that tripped the listing and filter further by contacts who have never opened or clicked. Because spam traps live in the unengaged portion of the list, removing those who have never opened or clicked will likely remove the individual contact that was responsible for triggering the blacklist. If the listing does not occur again, then the blacklist spam trap was successfully removed and mailing can resume as normal. All in all, the best way to remediate blacklistings is to mail responsibly in order to prevent the listings in the first place. By mailing to the core engaged audience (0-6 months) differently and in a different funnel with different content, segmentation, and frequency than those who are older than 6 months or so, blacklists and spam trap hits can be far less frequent.