Understanding Blocklists

What is a blocklist?

A blocklist is a list created and maintained to monitor email sending behaviors, to help email service providers (ESPs) and filtering products to more effectively block spam and fraudulent email from their customers' inboxes.

It's important to keep in mind that just because you see your domain or sending IP on a blocklist doesn't mean that's having a large effect on your email deliverability. It's certainly not a good thing, but there are a large number of blocklists and each recipient domain's email server system usually only checks one or two as a part of their email filtering protocol. Blocklists don't necessarily block your email - the receiving server's spam filter implementation would be where the block originates.

There are two different types of blocklists. IP-based blocklists contain lists of IP addresses deemed spammy, which are updated and checked in real-time. This type of blocklist is the most commonly used. The other type is domain-based, and lists of this type contain domain names contained in emails identified as spammy.

Some acronyms related to blocklists

What blocklists are reputable? What blocklists aren't reputable?

To address this, we want to first define what we mean by "reputable" here: reputable in the email blocklist world primarily means that the blocklist is actively maintained. Essentially anyone can create a blocklist, and as a result they don't all stay maintained over the years. You may do an internet search on a sending IP and find that it's on a blocklist, but it's important to check that listing's website. An IP could be listed on a blocklist which has been dormant and unused for years. In these cases, it's unlikely that blocklist is being used for filtering by any existing mail servers.

These are some commonly used and reputable blocklists:

• Spamhaus (SBL, CBL, XBL, DBL)
• Barracuda
• SpamCop (SCBL)
• Invaluement (ivmURI, ivmSIP, ivmSIP/24)
• URIBL
• SURBL

If you find references to other blocklists in your email stats or have any concerns about a blocklisting issue, please reach out to Support!

Spam Traps and other issues with a poor-quality list

Old, stale, or purchased lists often contain dormant email addresses turned into spam traps by blocklists and public ESPs. Hitting one or many spam traps on the same receiving domain is the quickest way to find your sending IPs or domain on a blocklist.

Maintaining a clean, updated, and permission-based list which you send to at least every 3-6 months is the best way to avoid having spam traps on your recipient list. Stale emails will run a higher risk of becoming spam traps, as well as receiving complaints.

Spam Complaints

If the percentage of recipients exceeds their filtering system's blocklist's threshold, those complaints would land the sender IP or domain on that blocklist. We don't know exactly what these thresholds are, but the best way to avoid being reported as spam is to maintain engagement with your recipient list so they don't believe your mail is unsolicited. Additionally, making sure to send only to permissions-based recipients list is very important for deliverability, reputation, and compliance with international spam laws such as CASL and CAN-SPAM.

How do I get delisted or removed from a blocklist?

Each blocklist has a different delisting process - some of them are automatic, some of them allow delisting by request, and others keep their processes a bit of a mystery to ESPs in order to maintain the integrity of their process.

When it comes to automatic processes, we can't directly address those to request delisting.

SpamCop, for example, has a 24 hour automatic delisting process. Often by the time we hear about an issue, that IP has already been delisted. Other blocklists with automatic delisting are a bit more close-lipped about their timeframes, and the only way to make sure delisting is possible is to identify and correct any poor sending behaviors which likely caused the listing.

Certain blocklists like Barracuda do allow us to request delisting. In these cases we need to provide a description of the cause of the listing and explain how we are seeking to rectify the issue or behavior.