A blacklist, or specifically a DNSBL (DNS-based Blackhole List) is a service operated by a party with the intent of restricting mail traffic from various domains to users who subscribe to that DNSBL. Criteria used to determine whether an IP address is to be listed is not standardized and is therefore up to the list maintainer to determine. The end result are lists that often use arbitrary or unreasonable criterion, sometimes even in conflict with those of other lists. Below we have compiled notes on various blacklists for your reference:

• SpamCop: Very accurate prediction algorithms, resulting in a near perfect rating when it comes to not blocking good mail, and an above average score for canning actual spam. Conservatively blocks CIDR ranges. ¹⁾
• FiveTen: Requires double opt-in list management for hosts. Blocks entire CIDR ranes (/24 and larger) quite liberally, resulting in on average twice the amount of good mail lost than actual spam blocked. ²⁾
• ivmSIP: This includes ivmSIP/24, which is a CIDR-block list similar to that of FiveTen. According to list maintainer Rob McEwen: “only my subscribers use my data, and most of them either don't use ivmSIP/24, or use that list for scoring instead of blocking.” Although ivmSIP is listed as a public DNSBL, as per the list maintainer, being listed here is going to have little impact on mail deliverability, unless the specific host you are attempting to deliver to uses this list to filter mail.
• Barracuda: Their spam scoring and listing methods are not widely understood (Barracuda intentionally obscures them) but it is known they use a reputation system similar to Senderbase. They do also employ honeypots (spamtraps) to discover spammers.

The best methods for staying off blacklists in general is to maintain a tight rein on users on your mailing/distribution lists and provide an easy way to opt-out of future mailings. Lists purchased from email harvesters are a sure-fire way to land yourself on most major blacklists and furthermore is in violation of our AUP. Best practice is to gather subscribers based on a double opt-in process, publish SPF and DKIM records, and ensure you have valid and matching forward and reverse DNS records. Details on mailing best practices are detailed in the article below.

Emailing Best Practices Guide

If you find that you are listed in a DNSBL and have attempted to follow the guide's recommendations, please email abuse@ndchost.com with your removal request and we will be happy to forward it to the appropriate DNSBL maintainers on your behalf.