What Are Email Blocklists (and How to Avoid Them)

May 16, 2023
Written by
Jesse Sumrak
Opinions expressed by Twilio contributors are their own
Reviewed by
Ayanna Julien
Opinions expressed by Twilio contributors are their own

What Are Email Blocklists (and How to Avoid Them)

Most email marketers have found themselves on at least one IP or domain blocklist at some point. If you find yourself on a blocklist, it’s a good idea to assess your address collection practices, evaluate your sunsetting policy, and request a delisting from the relevant blocklist removal form (if available).

The impact of having your IP or domain on a blocklist can range from being a small annoyance to a complete showstopper for your email deliverability. Because deliverability is the primary concern for most email marketers, it’s crucial to understand how blocklists function and what you can do as a sender to reduce your risk of being blocklisted.

What is an email blocklist?

First, let’s start with a basic definition: what is an email blocklist? An email blocklist is a record that contains IP addresses or domains flagged by spam filters. Servers use this information to determine which addresses to block—either by diverting directly to a spam folder or by disallowing the send to occur at all. 

In simple terms, a blocklist is a barrier between you and your email recipients. While some email blocklists have a bigger impact on your deliverability than others, your email marketing success depends on tracking metrics and ensuring good open rates. 

How an email blocklist works

To help you understand how a blocklist works, check out the diagram below.


For the most part, spam traps fall into 1 of 3 categories:
  1. Recycled spam traps: These email addresses were once valid but have been dormant long enough and haven’t engaged with any email in a long time. The receiving server typically refuses (bounces) the messages sent to these addresses for a year or more before reactivating them as spam trap addresses.
  2. Typo traps: These email addresses usually end up on recipient lists due to user error. For instance, typing example@hitmail.com instead of example@hotmail.com. Similar to recycled spam traps, these addresses never open or click any of the messages received. The anti-abuse community believes sending excessive amounts of mail to typo traps is indicative of poor list acquisition practices and poor list hygiene.
  3. Pristine traps: These fake email addresses and domains, created by ISPs or anti-spam organizations, never actively receive email for the sole purpose of capturing a spammer. Pristine traps commonly end up on mailing lists when senders purchase, rent, or scrape addresses, and are most likely to put a sender on blocklists.
Note: Some email blocklists also list domains and IP addresses based on user-generated feedback and manual reporting of unsolicited emails.

How to check if you’re on an email blocklist

There are a lot of blocklists, but a good starting point is to perform an email blocklist check to see if your IPs or domains are on any of these popular lists:
  • Barracuda Reputation Block List (BRBL): BRBL is a free DNS blocklist (DNSBL) of IP addresses known to send spam.
  • Cloudmark: Cloudmark’s impact can be significant if Proofpoint’s influential blocking service catches you with malware on your computer, a compromised user account, or dated subscriber lists.
  • Invaluement: Invaluement's anti-spam DNSBL blocks elusive types of spam where the sender sends unsolicited bulk email and escapes traditional detection methods.
  • MXToolbox: MXToolbox shows you whether your domain or IP address is on a blocklist and can perform checks on your DNS to determine its configuration.
  • MultiRBL: MultiRBLis a free multiple-DNSBL service that cross-references other blocklists by IPv4, IPv6, or domain.
  • SpamCop: The SpamCop Blocking List (SCBL) collects IP addresses that had mail reported as spam by SpamCop users.
  • Spamhaus: The Spamhaus Project maintains several DNSBLs as part of its effort to identify and track spam sources and provide anti-spam protection. 
  • SURBL: SURBL, unlike most lists, is not list of message senders. Instead, these are lists of websites appearing in unsolicited messages.

How to reduce your risk of getting blocklisted

Want to know what to do when your email gets blocked? Fortunately, there are plenty of corrective actions you can take before you wind up on an email blocklist.

But if you want to know how to avoid email blocklists, your answer is reputation monitoring—the key to maximum email deliverability. By closely monitoring your complaint rates, you can prevent delivery failures proactively. Check your statistics with each campaign deployment and look for delivery dips and low engagement rates. You also need to be mindful when adding new subscribers to your email list to ensure you’re not the victim of spam traps.

Keep in mind the best solution differs for every business model. Here are several actions you might take:

1. Confirmed opt-in

Before adding a new recipient address to your active mailing list(s), send a confirmation email.

  • Adds only valid, engaged recipient addresses to your mailing list
  • Prevents typo traps, recycled spam traps, and pristine trap addresses on your list
  • Increases open and click rates (because you’re sending emails to recipients who explicitly asked to receive them!)
  • Reduces spam reports and unsubscribes
  • Improves your return on investment (ROI) for each message sent
  • Reduces marketing email opt-ins (possibly)
  • Causes extra “friction” in the sign-up process

2. Engagement-based sunsetting

Remove email addresses from your mailing list if these don’t open or click a message in a certain period. This period varies depending on factors like your industry and sending frequency. Pros:
  • Removes “dead weight” from your email list
  • Improves sending reputation by only sending to recently engaged recipients
  • Improves ROI per message sent
  • Reduces spam reports and unsubscribes
  • Reduces exposure to recycled spam traps and typo traps significantly
  • Improves brand reputation by reducing the number of emails sent to people who don’t want them
  • Requires significant technical requirements to determine who is engaged
  • Decreases list size
  • Leaves money on the table potentially (e.g., What percentage of long-term, nonengaged email recipients eventually convert to a sale?)

3. Real-time address validation 

Check email addresses at the point of sign-up for validity and common typos. Pros:
  • Reduces common domain typos
  • Reduces pristine spam trap hits
  • Reduces instances of fake email addresses
  • Reduces “friction” compared to confirmed opt-in
  • Causes new sign-ups to often react positively to this feature (e.g., “Wow they caught my typo!”)
  • Incurs licensing costs if using third-party service
  • Has engineering and technical requirements when building your verification logic

Achieve better deliverability with Twilio SendGrid 

Blocklists don’t exist to make life more difficult for email senders. These are here to help mitigate spam and keep recipients happy.

Remember that the purpose of email blocklists is to prevent email from landing in inboxes that didn’t recently opt in to receive email with explicit and informed consent.

This means that no single preventative measure will guarantee zero blocklistings.

But it also means that if you do everything in your power to send email that people want, it’ll make the process of getting delisted that much easier!

At Twilio SendGrid, getting your emails delivered to your recipients’ inboxes is our top priority. Our user-friendly dashboard, domain, and IP blocklist monitoring, and real-time email alerts keep you on top of your reputation score to discover deliverability issues. Learn more, then get started for free.

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