Klaviyo IP Blocklisted or Blacklisted: What to Do When Your Mail Gets Listed

Blocklists (also called blacklists, though "blocklist" is preferred) are lists of IPs and domains associated with spam, phishing, or other abuse. Major ISPs consult multiple blocklists when deciding whether to accept mail. Being listed means the mail is rejected or filtered to spam at any ISP that uses the list. For Klaviyo senders, this produces sudden, broad delivery failures that can affect the full audience at affected providers.

This article walks through how to recognise a blocklist issue, the major blocklists that matter, and the specific steps for resolution, including the critical distinction between IP-level blocklists (Klaviyo's responsibility) and domain-level blocklists (yours).

How to Recognise a Blocklist Issue

The signs of a Klaviyo IP or domain being blocklisted:

Sudden mass rejections. Bounce rate spikes abruptly (not gradually) and the rejections are concentrated at specific ISPs. Other ISPs may be unaffected.

Bounce codes citing blocklists. 550 5.7.1 responses often reference the blocklist by name ("listed on Spamhaus SBL," "SURBL listing," "UCEPROTECT").

Recent send of something unusual. Blocklists usually catch specific sending behaviours: a send to a spam-trap-heavy segment, an unsolicited list, a phishing-like content pattern, or a volume spike far outside baseline.

Check results on blocklist lookup tools. Running your sending domain, tracking domain, or Klaviyo's IPs (where accessible) through MXToolbox, HetrixTools, or direct blocklist lookups confirms the listing.

A blocklist listing is urgent. While the listing is active, every send to the affected ISPs fails. Resolution is usually possible within hours to days depending on the specific blocklist and the cause.

The Major Blocklists and Who Owns Them

Spamhaus (spamhaus.org)

The most widely used blocklist ecosystem. Multiple lists with specific purposes:

Spamhaus SBL (Spamhaus Block List): IP-level. Listings are for senders associated with sustained spam activity.

Spamhaus XBL (Exploits Block List): IP-level. Listings are for compromised machines sending spam.

Spamhaus DBL (Domain Block List): Domain-level. This is the one Klaviyo customers can and should check for their own sending domain and tracking domain.

Spamhaus CSS (Composite Snowshoe Sender): IP-level. Listings are for senders using snowshoe spamming patterns.

Spamhaus PBL (Policy Block List): IP-level. Listings are for IPs that should not be sending mail at all (residential IPs, dynamic IPs).

Spamhaus listings are consulted by major ISPs including Gmail, Microsoft, and Yahoo. A Spamhaus listing has the broadest impact.

SURBL (surbl.org) and URIBL (uribl.com)

URL-based blocklists. These list domains that appear in spam URLs, which means they primarily affect senders whose tracking domains are listed. For Klaviyo customers, your tracking domain can end up on these lists if it has been compromised or previously used for spam.

SURBL and URIBL listings are consulted in content-based filtering. Being listed does not cause SMTP rejection but does cause spam-folder filtering.

UCEPROTECT (uceprotect.net)

An aggressive blocklist with several levels. Level 1 is IP-specific. Levels 2 and 3 are network-range listings that can catch entire subnets because of a single bad actor. UCEPROTECT is less widely consulted than Spamhaus but still influences some filtering decisions.

Microsoft Internal Blocklists

Microsoft maintains internal blocklists (not public, not visible to senders) that affect Microsoft delivery specifically. A block at a Microsoft internal list shows up as S3150 or similar codes. Resolution requires working with Microsoft Sender Support, which Klaviyo's deliverability team handles for customer IPs because Klaviyo owns the IPs.

Other Blocklists

Many smaller blocklists exist (Barracuda, SORBS, PSBL, and others). Most do not significantly affect major ISP delivery. MXToolbox checks dozens of blocklists in one lookup; focus on the major ones that affect Gmail, Microsoft, and Yahoo.

What Klaviyo Customers Can and Cannot Do About Blocklists

This is the most important practical point for Klaviyo senders:

You can check and delist your own domain-level assets:

• Sending domain against Spamhaus DBL, SURBL, URIBL.
• Tracking domain against SURBL and URIBL (most common listing).
• Any subdomain you use for sending.

You cannot check or delist Klaviyo's IPs:

• Klaviyo owns the shared-pool IPs and the dedicated IPs they assign.
• Blocklist delisting requires the IP owner to interact with the blocklist. Klaviyo does this.
• When you escalate an IP-level listing to Klaviyo deliverability support, they can see the listing, verify the cause, and interact with the blocklist. You cannot do this directly.

Escalation to Klaviyo for IP-level issues should include: specific bounce codes and full rejection messages (which often name the blocklist), the affected ISPs, the time the issue started, and any recent changes in sending behaviour.

Delisting Your Sending Domain

If your sending domain or tracking domain is listed, the process:

Spamhaus DBL Delisting

1. Verify the listing at spamhaus.org/lookup.
2. Identify why you were listed. Usually it is a specific email campaign with spam-characteristics, a compromised domain, or a tracking-domain misuse by another party.
3. Fix the underlying cause before requesting delisting.
4. Submit a delisting request through Spamhaus's removal form.
5. Delisting usually happens within hours to a day after the cause is addressed.

SURBL/URIBL Delisting

1. Verify listing at surbl.org/surbl-analysis or uribl.com/lookup.
2. Identify the cause. Often a historical listing from a previous owner of the domain, or a tracking-domain hit.
3. Submit delisting request.
4. Resolution typically within hours.

UCEPROTECT Delisting

1. UCEPROTECT delistings typically resolve automatically after 7 days without further listings.
2. Expedited delisting is available for a fee, which many deliverability professionals consider questionable practice. Waiting the 7 days is usually fine.

What Triggered the Listing

Before submitting a delisting request, identify what triggered it. Otherwise, you will be relisted. Common triggers:

• Sending to a large batch of spam-trap addresses (usually from a purchased or scraped list).
• A complaint-rate spike above the blocklist's threshold.
• Tracking-domain abuse: the tracking domain previously owned by another entity that used it for spam.
• Content patterns associated with phishing or malware.
• Volume pattern that matches snowshoe spam (many IPs, low per-IP volume, no clear legitimate use).

Addressing the trigger before delisting is essential. Submitting a delisting without fixing the cause gets you relisted within days, often with harder consequences.

While the Listing Is Active

Until delisting completes:

• Pause sending to the affected ISPs if possible. Every attempt fails and extends reputation damage.
• Do not run "catch-up" campaigns after delisting. Resume at normal volume to avoid reputation being interpreted as suspicious post-delisting behaviour.
• Document the specific campaigns, segments, or sends that preceded the listing. This helps avoid repeating the trigger.
• For IP-level listings, maintain communication with Klaviyo deliverability support on delisting progress.