So You’ve Been Blocklisted. Now What?

If there were one word that sent email marketers spiraling, it would be blocklist. Getting your sending IP or domain flagged as a source of spam would be anyone’s worst nightmare. Especially when you consider how every email that doesn’t land in the inbox is money left on the table.

We covered the blocklist basics in our Monster Guide, so today we’re diving deeper into the various types of blocklists, ranked by severity level. Then we’ll provide step-by-step instructions for mitigating listings.

Because as anxiety-inducing as landing on a blocklist might be, it’s not the end of the world. With the right deliverability platform and people around you, you can take a deep breath and kick those sending scaries to the curb.

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Blocklists by Severity Level

Having your IP or domain appear on a blocklist can severely impact your email deliverability. But not all blocklists are created equal.

Here is a breakdown of the most common blocklist types and their relative severity.

High Severity Blocklists 🔴

The following blocklists are the email equivalent of Monopoly’s chance card: “Go directly to jail. Do not pass Go. Do not collect $200.”

These blocklists are widely used by major ISPs and spam filters, so being listed will almost certainly result in emails going missing or being sent straight to spam.

Spamhaus (ZEN, SBL, XBL, PBL, DBL)

• Impact: Severe. Used by Gmail, Yahoo, Outlook, and others.
• Delisting: Requires a formal removal request and often corrective action.

Barracuda (BRBL)

• Impact: High. Frequently used by corporate filters and mail appliances.
• Delisting: Removal request required via form; they may recheck periodically.

Proofpoint ET / Cloudmark CSI

• Impact: High to critical. Filters both enterprise and consumer inboxes.
• Delisting: Based on reputation improvements; limited manual recourse.

Invaluement

• Impact: High. Used by multiple spam filtering services.
• Delisting: Removal requires proving you’ve resolved the issue.

Moderate Severity Blocklists 🟠

These blocklists are still in use across various filtering platforms, but their impact is usually isolated to specific mail systems or regions.

SpamCop

• Impact: Moderate. Often used as one of several filtering criteria.
• Delisting: Automatic after 24 hours if spam reports stop.

UCEPROTECT (Level 1) (Level 2) (Level 3)

• Impact: Moderate to low. Not widely respected; controversial inclusion policies.
• Delisting: Level 1 self-removes; Level 2 and 3 require payment. You can choose to pay for delisting, but paying to be delisted is why this blocklist is not widely respected.

Low Severity or Niche Blocklists 🟡

The lowest severity blocklists are often used in niche environments or to supplement other filters. They may not affect inboxing unless combined with other issues.

HostKarma

• Impact: Low. Some use it in scoring models.
• Delisting: Email removal request.

Lashback UBL

• Impact: Low to moderate. Focused on unsubscribe compliance.
• Delisting: Typically automated after complaint reduction.

NoSolicitado / PSBL

• Impact: Low. Used primarily by smaller regional providers.
• Delisting: Easy or automatic.

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Understanding the Impact of Blocklists on Deliverability

Blocklistings often occur when emails are sent to subscribers who did not want or expect them.

Getting blocklisted typically happens when:

• Too many recipients mark messages as spam
• Emails are sent to inactive or outdated contacts
• Cold, purchased, or partner data is mailed without strong consent controls
• Unsubscribe options are hard to find or don’t work quickly

Listings can happen fast—sometimes within hours of a problematic send.

If you find yourself on a blocklist and want to know how your deliverability is being affected, you can pull your bounce logs for the days you are blocklisted. You can use SQL queries to search for any variation of:

• “blocked using zen.spamhaus.org”
• “IP address [xxx.xx.xxx] is blocked by Spamhaus”
• “550 5.7.1 IP Block-listed in RBL bl.spamcop.net”
• “Your IP address has been blocked by Barracuda's BRBL (Barracuda Reputation Block List).”
• “550 5.7.1 UCEPROTECT Blocklist: [Sending IP Address] listed as a source of spam.”
• “550-5.7.1 Your IP address is currently blacklisted by HostKarma [Source IP Address]”
• “Lashback UBL”
• “NoSolicitado”

This will identify mailbox providers that block using a particular blocklist. Inbox Monster’s Subscriber Insights show how many of your subscribers are blocked, and what percentage of your list is affected. How high these figures are will indicate the impact on your overall deliverability.

7 Steps to Mitigate Listings

So you found out you’ve been blocklisted. Now what?

Here are seven steps you can take immediately to stop things from escalating from bad to worse:

1. Pause the campaign that’s causing complaints. Whether you’re experimenting with email creative, using clickbaiting subject lines or ramping up frequency, your first step is to identify the primary culprit and stop the bleeding.
2. Suppress the affected audience. The biggest risk of blocklisting comes from mailing to old or inactive subscribers, especially when consent is unclear and the data is cold or appended.
3. Reduce volume to recent engagers only. Inactive recipients are far more likely to report spam. We recommend suppressing users with no engagement for 90-180 days. Sending re-engagement campaigns to smaller audiences is much safer than mailing large, quiet lists without guardrails.
4. Allow time for complaints to stop. In many cases, a blocklist auto-delists within 24–48 hours once behavior improves.
5. Request bounce logs for the blocklisted dates. You can use the queries from the previous section to identify which IPs and domains were affected.
6. Dive deeper with a deliverability platform like Inbox Monster. Our Subscriber Insights allow you to see what percentage of your list is impacted.
7. Sunset recipients who never respond. Over-mailing unengaged users is your biggest risk factor, so say goodbye to those who have had plenty of opportunities to engage but have very low or no click rates.

Blocklistings often follow sudden changes—like new sending domains or volume increases—so slowing down and making gradual adjustments can make all the difference.

Becoming Blocklist Free

If your campaign performance is dropping, it might indicate that emails are being blocked or routed to spam by mailbox providers.

Blocklistings usually signal data hygiene issues, rather than technical problems, so the most effective fix is better targeting and easier opt-outs.

And with the right technology, you can demystify deliverability issues and get off blocklists for good.

If you’re interested in learning how Inbox Monster can keep you free from blocklists, schedule a demo of our deliverability platform today.

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