What are blacklists?

Basically a blacklist is a list of IPs suspected of sending spam. DNSBL (DNS-Based Blackhole List) and RBL (Real-time Blackhole List) list these IPs. They are used to ideally prevent the transmission of spam to recipients. Keep in mind that it's not the blacklist providers that are blocking your mail. It's the ISPs using this information to determine if your mail will be accepted. In other words, blacklists influence the behavior of ISP recipient servers.

Almost anyone can create a blacklist and there are a lot of them out there, you can check it on MultiRBL. With so many blacklists out there, not all are considered equal by ISP's, and since ISP's don't treat them all equally, neither should you.

How do you know which blacklists matter?

Luckily for you, we've put together a list of some blacklists that we think are worth paying attention to:

• Spamhaus 
• Barracuda
• SpamCop
• Sorbs
• Invaluement
• CBL
• SURBL multi
• URIbl black

Elastic Email actively monitors blacklists (including, but not limited to the ones listed) and if one of our servers ends up on one of them, our Delivery Experts investigate the issue and get us removed as quickly as possible.

In addition to the more influential providers, there are many smaller blacklists. Most of them, however, are unlikely to have a lot of impact on major ISPs and won't cause bounces or other delivery issues.

How do blacklists know when to list an IP or domain?

All providers use a variety of ways to measure whether or not mail is wanted or unsolicited, but most include using some combination of spam traps and feedback loops. Spam traps are fake or old email addresses that have been re-purposed to see if anyone sends it to them. If they do, then the owners of those spam trap addresses know that the domain or IP sending mail to that address is sending unsolicited mail.

Feedback loops, otherwise simply known as recipient responses to email messages is another way that you can end up blacklisted. If there are too many direct abuse complaints (people mark your mail as spam or use other channels to report unwanted mail) then blacklist providers suspect unwanted mail and domains or IPs associated with that mail, and sometimes even mail that looks similar in nature.

How do I know if I'm blacklisted? What do I do if I am?

You can run a quick scan to see if you're on any major blacklists, here are a couple of tools you can use:

MultiRBL

MXToolBox

IPLocation

In most cases, you simply need to visit the site that's blacklisted you and use their de-list request forms. Usually, this effort will result in clearing your domain or IP within 24-72 hours. In some cases, however, you just have to stop sending the suspect email or remove what appears to be a spam trap address and then just wait. Once the provider has seen that the spam has ceased, they are likely to clear the IP.

What's the most important lesson to learn from blacklists?

Don't send unwanted mail. Don't send mail to email addresses that have not explicitly given you permission to mail them, in other words - don't purchase data or lists. Use double opt-in subscription methods, keep your lists up to date and stop sending mail to recipients that don't open it.

List hygiene is the most important way to prevent blacklistings followed by creating excellent and relevant email marketing content. Just those simple steps will prevent you from dealing with the frustration that can result from a blacklisting.