The Best DNS Blacklists – Not All Blacklists Are Created Equal

Written by Nick Schafer

Categories: Best Practices

3 minute read time

At Mailgun, we often get emails from customers letting us know about IPs appearing on blacklists. We want to set the record straight on this. Not all blacklists are created equal. Some blacklists you should care about, but there are others that you shouldn’t lose any sleep over.

What is a blacklist anyway?

Before we run through the different types of email blacklists, let’s cover what blacklists are exactly. DNSBL (DNS-Based Black List) and RBL (Real-time Black List) are lists of IP addresses that are suspected of sending spam (like phishing emails and scams) or housing spammers and are used to prevent unwanted email messages from reaching unsuspecting recipients. 

An important thing to mention is that blacklists don’t actually block your messages but rather the actual mailbox providers.

In this way, blacklists are removing the spam sources, but the spam itself. Blacklists use resources like SpamAssassin to help grade or score IPs, and then the DNSBL will blacklist based on their own parameters. These providers use this information from various blacklist services along with internal metrics to make decisions on whether or not to block a message. This is noteworthy because just being listed on a blacklist isn’t necessarily going to cause you problems with your deliverability.

There are a lot of blacklists out there, and we mean a lot: check here to see how many we are talking about. These are created by large, reputable businesses, as well as little lesser-knowns like surbl, and even fraudsters. Since pretty much anyone can create a blacklist, they aren’t all treated the same by ISPs (think Gmail and Yahoo), so you shouldn’t treat them the same either.

Which blacklists matter?

So how do you decipher between the reputable blacklists and the fraudsters? Don’t fret. We’ve got you covered as we have compiled a list of the ones we pay the closest attention to:

  • Spamhaus – Spamhaus.org is one blacklist you don’t want to mess around with. The Spamhaus block list (sbl) is used by many ISPs around the world including Yahoo, Gmail, and Microsoft. If you end up on Spamhaus.org, you have significant problems. Spamhaus also has several subsets of block lists like xbl, dbl, and pbl that are important to be aware of as well.
  • SpamCop – SpamCop determines the origin of unwanted email based on reports from users of their service and passes it along to the relevant Internet service providers.
  • Invaluement – Invaluement is a compilation of 3 commercial anti-spam blacklists:
    • ivmURI – A domain-based blacklist that lists domain names used as clickable links in spammy messages.
    • ivmSIP – An IP-based blacklist that lists IPs that are showing signs of sending large amounts of spam or solely sending spam.
    • ivmSIP/24 – An IP range blacklist that blocks entire ranges that are emitting a large number of spam messages.
  • Barracuda – Barracuda is a free DNSBL of IP addresses known to send spam. Barracuda Networks fights spam and created the BRBL to help stop the spread of spam. Many corporations use Barracuda too. If you send B2B messaging, be on the lookout for this one.
  • Lashback (Unsubscribe Black List) – Lashback empowers organizations to identify and block the very worst unsubscribe abusers: those that send email to addresses which have been harvested from suppression lists.
  • PSBL (Passive Spam Block List) – PSBL is an easy-on, easy-off blacklist that does not rely on testing and should reduce false positives because any user can remove their ISP’s mail server from the list.
  • sorbs.net – Sorbs.net (Spam and Open Relay Blocking System) is another free DNSBL of IP addresses. When it comes to this one, you shouldn’t be worried. Sorbs.net has a ton of IPs on its blacklist, and if you’re on this one, you likely won’t see any effect to your message delivery.

 

Find the Problem

Finally, a word of caution: just because we are saying you can treat some listings differently, the fact remains that something caused the listing in the first place. As a result, you should always be maintaining best practices when it comes to your email program. Oftentimes, blacklisting happens because you send a message to a spam trap that is being monitored by one of these blacklist operators.

If you do happen to get listed, take a close look at your list collection practices. Make sure you are incorporating confirmed opt-in to your sign up process, and lastly always keep up with your list hygiene. You might also want to check how you’re sending, like from shared server versus your own SMTP. Once you’ve done the work to identify the cause of a listing, feel free to put in a request removal (aka, delisting) from the blacklist. If you don’t take necessary measures to prevent a re-listing you will get listed again, guaranteed. Frequent recurrences could lead to future removal requests being rejected, so fix the problem ASAP before you’re listed again. Plus, your email service provider will be happy to see you off of the blacklist for good, so everyone wins.

If you have any questions on blacklists or spam lists, improving your overall email deliverability or anything else email-related, don’t hesitate to reach out to our team of email experts at Mailgun and we will be happy to help!

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Modified on: August 22, 2019

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