About CBL (Composite Blocking List)
The Composite Blocking List (CBL) at cbl.abuseat.org is a fully-automated DNSBL operated by abuseat.org that lists IPs running known email exploits — open relays, open proxies, compromised mail servers, malware-infected hosts emitting spam, and similar abuse indicators. CBL is one of the major feeds into Spamhaus XBL, so a CBL listing typically surfaces as an XBL hit when queried via Spamhaus ZEN. Unlike Spamhaus SBL (manual research), CBL is entirely automated based on Spamhaus's probe infrastructure.
Most mail servers that query Spamhaus XBL effectively also query CBL through the integration. Direct cbl.abuseat.org queries appear in many independent Postfix and Exim DNSBL chains, particularly in ISP and hosting-provider mail platforms. Practical impact is similar to XBL — SMTP-time rejection at strict receivers, soft-fail at scoring-based receivers.
Automated probes detecting open relays, open proxies on standard or non-standard ports, compromised mail-sending applications (typically vulnerable PHP scripts), botnet activity, or other technical evidence of exploitation. The probe is the primary signal — manual reports do not typically generate CBL listings.
How To Get Delisted From CBL (Composite Blocking List)
- 1
Run the CBL lookup at abuseat.org
Open https://www.abuseat.org/lookup.cgi and enter the listed IP. The result page shows whether the IP is listed and includes a detailed description of what the probe detected (e.g. 'open SMTP relay on port 587 accepting unauthenticated forwarding', 'IP responding as HTTP open proxy on port 8080', 'IP sending traffic consistent with botnet pattern X').
Note: Read the description literally — it tells you exactly what to fix. The CBL is one of the most transparent DNSBLs in terms of evidence disclosure.
- 2
Reproduce and remediate the exploit
Use the description to find the issue. For 'open relay' listings: test your mail server with telnet or swaks from an external network — if you can relay mail to an unrelated domain without authenticating, the relay is open. Configure SMTP AUTH to be required for all forwarding. For 'open proxy' listings: scan your IP from outside for HTTP/SOCKS proxies on standard ports — kill the open proxy. For 'compromised host' listings: scan for malware, audit recent CMS installations for known vulnerabilities (especially WordPress, Joomla, Drupal with outdated plugins or themes).
Note: The CBL re-tests after you submit removal — half-measures fail because the probe will still find the exploit.
- 3
Submit removal via the CBL self-service form
On the CBL lookup result page, click the 'Remove' button. The form is minimal — no contact email required, no detailed description, no manual review. You're confirming that the exploit has been closed. The system immediately runs an automated probe to verify.
Note: Self-removal is rate-limited per IP — typically 1-3 removals within a short window before you're forced into a manual queue.
- 4
Let the automated probe verify and complete removal
The probe takes seconds to minutes. If the exploit is verified-closed, the IP is delisted from CBL within 1-2 minutes. If the probe finds the exploit still active, the removal fails and you'll be told exactly what's still detected. Re-fix and try again.
Note: Probes use multiple Spamhaus-controlled IPs. Allowlisting probe IPs is detectable and triggers immediate re-listing.
- 5
Verify XBL and ZEN propagation
Once CBL clears, Spamhaus XBL (which is fed by CBL) updates within an hour. Check https://check.spamhaus.org to confirm both XBL and ZEN are clean. Receivers refresh DNSBL caches within 1-4 hours.
Note: Some receivers cache reputation signals longer — Microsoft 365 in particular. Allow 6-12 hours before assuming full deliverability recovery.
- 6
Resume sending and prevent recurrence
Restart sending at 10-20% of previous volume for 48 hours, then ramp gradually. Audit and lock down the systems that caused the listing. Common causes for cold senders: shared mail infrastructure where another tenant has a compromised application — the durable fix is dedicated IPs on isolated infrastructure where no other tenant can re-trigger the listing.
Note: ColdRelay's isolated Azure tenant architecture removes the shared-infrastructure CBL risk by design — the closed M365 outbound path has no open-relay, open-proxy, or vulnerable-PHP surface.
Operational Details
1-12 hours total. Self-removal verification: 1-2 minutes. XBL propagation: 1 hour. Receiver-side DNSBL cache refresh: 1-4 hours.
Re-detection of the exploit by CBL probes. Self-removal limits force the IP into manual review with 24-72 hour turnaround after multiple failed automated removals.
Lookup and removal: https://www.abuseat.org/lookup.cgi. Manual queue contact: https://www.abuseat.org/contact.html (only for cases where automated removal repeatedly fails despite remediation).
CBL (Composite Blocking List) And Cold Email
CBL listings for cold senders almost always come from shared mail infrastructure — a cheap VPS, a shared hosting plan, or a budget mail server where another tenant runs a compromised WordPress site or open relay. Because CBL is probe-based and per-IP, the listing applies to the IP itself regardless of who's using it. Your own sending can be perfectly clean and you'll still be CBL-listed if your IP neighbour has an exploit. The durable fix is dedicated IPs on isolated infrastructure: ColdRelay assigns each customer dedicated sending IPs inside their own isolated Azure tenant, and the M365 outbound mail path has no open-relay/open-proxy/PHP-exploit surface for probes to find. Combined with the per-mailbox 2-emails/day cap (which keeps the volume profile far from botnet-pattern detection), the structural CBL risk drops to effectively zero.
Frequently Asked Questions
How long does CBL removal take?
1-12 hours total when the underlying exploit is actually fixed. The CBL self-service removal at abuseat.org/lookup.cgi runs an automated probe to verify, which takes minutes. XBL propagation adds about an hour. Receiver-side DNSBL cache refresh: 1-4 hours.
What's the relationship between CBL and Spamhaus XBL?
CBL feeds into Spamhaus XBL. Most XBL listings ARE CBL listings, and the CBL self-removal tool at abuseat.org/lookup.cgi is the way to delist from both simultaneously. They share infrastructure and detection logic but are presented as separate brands — abuseat.org operates the CBL, Spamhaus aggregates it into XBL.
Why does the CBL keep saying my IP is still infected after I fixed the issue?
The probe is finding something. Common gotchas: an open relay where SMTP AUTH was added but a legacy port still accepts unauthenticated relay; a WordPress plugin that was disabled but not deleted (vulnerable code still loadable); an old DNS record pointing to a compromised host you forgot about. Read the probe's description carefully — it tells you exactly what it's finding.
Can I just send to a different IP if CBL won't clear?
Switching IPs avoids the listing but doesn't address the root cause. If your underlying infrastructure has the exploit (open relay, compromised host, vulnerable web app), the new IP will be CBL-listed within hours. The only durable approach is fixing the exploit, not changing IPs.
Does ColdRelay infrastructure ever get CBL-listed?
Structurally, no — each customer is on a dedicated isolated Azure tenant with dedicated sending IPs. The Microsoft 365 outbound mail path has no open-relay, open-proxy, or vulnerable-PHP surface for CBL probes to find. The infrastructure surfaces that drive CBL listings simply aren't present in the M365 outbound architecture.
Is CBL self-removal really automated with no contact info required?
Yes — that's one of CBL's most operator-friendly features. The self-removal form takes the IP, you click submit, and the system runs an automated probe. No contact email, no description, no manual review. As long as the exploit is verified-closed, removal completes in minutes.
Will paying anyone help speed up CBL removal?
No — and beware of anyone offering paid CBL removal. CBL has no paid path; the self-removal at abuseat.org is free and immediate when the exploit is closed. Anyone offering 'fast CBL delisting' for a fee is running a scam.