IP and Domain Blocklist Checks: Are You Listed and How to Fix It

When mail stops landing or links get blocked, it’s often a reputation problem, not a server outage.

The fastest triage is to check whether your sending IP and your domains are on well-known blocklists, then fix the root cause before you ask anyone to delist you.

This is a practical, operator-grade playbook: how blocklists work, how to check them safely, what common error codes signal, and how to request delisting without making things worse.

Understanding IP and Domain Blocklists

An IP blocklist is a DNS-published dataset that lists internet addresses observed sending or relaying unwanted mail or showing exploit behavior; a domain blocklist flags domains used in headers or URLs that receivers have seen in abusive traffic.

Common Triggers for Listings

Frequent triggers include compromised web apps that send spam through your mailer, a leaked API credential used to relay through your SMTP provider, poor list hygiene and high complaint rates, snowshoe-style sending from new dedicated IPs without warmup, open relays or proxies, missing or mismatched reverse DNS, and broken authentication such as SPF or DKIM failures.

Major Operators You’ll Encounter

At the IP level, Spamhaus operates SBL, XBL, PBL, and CSS that are commonly queried via ZEN; Barracuda runs the BRBL; Cloudmark maintains CSI; and large mailbox providers run their own systems and escalation paths, such as Microsoft’s delist portal and Gmail’s sender contact/mitigation process.

A Note on SORBS

SORBS shut down in June 2024 and no longer provides reputation data, so if you still see it in filters or tooling, remove it from checks and defenses and focus on active operators.

Are You Listed? Fast, Safe Checks

Start with the exact bounce or SMTP transcript and extract the error code and the receiver’s reference link; that link usually points to the specific operator and reason for blocking, which keeps you from wasting time on generic lookups.

Check your sending IP against Spamhaus ZEN and your sending and click-through domains against the Spamhaus DBL; if the bounce cites Barracuda or Cloudmark, use their lookups as well, and if the block is provider-specific (Microsoft, Gmail, etc.), follow the instructions in the non-delivery report and the provider’s postmaster documentation.

Validate fundamentals before you request delisting: the IP’s PTR (reverse DNS) must exist, the hostname must forward-resolve back to the same IP (forward-confirmed reverse DNS), and your SMTP HELO/EHLO name should match that hostname; SPF, DKIM, and DMARC must be published and pass for your traffic.

Reading Common NDR Patterns

Microsoft often returns 550-series errors that reference a delist path; codes 5.7.606–649 typically point to the delist portal, while 5.7.511 instructs you to email their delist team and can’t be fixed via the portal; Cloudmark rejections reference CSI with a link to their remediation form; Spamhaus-related bounces include a reference URL indicating which dataset triggered (SBL, CSS, XBL, PBL, or DBL for domains).

Provider Nuances in 2025

Gmail no longer exposes IP and domain reputation dashboards in Postmaster Tools v2, so you focus on compliance (SPF, DKIM, DMARC, one-click unsubscribe, low spam rate) and can submit a sender contact/mitigation request if you meet the guidelines and believe filtering is incorrect; there isn’t a general-purpose “delist button.”

Root Cause First: What to Fix Before You Delist

Quarantine nonessential sending, especially bulk mail, and stop tracking links that point through brand-new domains or shared shorteners; these patterns often correlate with domain listings.

Patch and scan any host that can send through your mail path, rotate API keys, update MTA and queueing software, and disable unauthenticated submission you find during the audit.

Set or correct reverse DNS so the IP has a single, stable PTR pointing to your mail hostname; confirm a forward lookup of that hostname resolves to the same IP, and configure your MTA to announce that name in the greeting.

Publish SPF that authorizes only current senders, sign all traffic with DKIM using strong keys, and enforce DMARC with alignment and reporting so you can see failure rates and unauthorized sources.

Clean lists: remove role accounts, bounces, and long-term inactives; honor unsubscribes immediately; suppress addresses that mark you as spam; and monitor complaint and bounce rates so they trend down before you ask for delisting.

IP Versus Domain Listings: Different Signals, Different Fixes

An IP listing usually reflects observed sending behavior such as high complaints, trap hits, malware, or snowshoe patterns, so fixes center on traffic quality, authentication, and host hygiene; a domain listing often implicates the URLs in your content, so you may need to replace landing pages, fix redirects, remove shady shorteners, and ensure the domain has clean history.

Shared Versus Dedicated Infrastructure

On shared IPs your control is limited, so you may need your provider’s abuse team or a cleaner pool; on dedicated IPs you own the reputation and must warm volume gradually and keep complaint and bounce rates low while you build history.

Requesting Delisting Without Backfiring

Spamhaus requires that the abuse is resolved before removal; include a specific summary of what happened, what changed, and how you’re monitoring to prevent recurrence, and expect the request to be declined if symptoms persist.

For Microsoft, follow the exact NDR guidance: use sender.office.com for 5.7.606–649 cases, and for 5.7.511 email the designated address with full NDR details; keep traffic paused or throttled until you see improvement.

Cloudmark CSI accepts remediation requests through their portal; provide headers, error samples, and a timeline of fixes; Barracuda BRBL provides a removal request form and reviews cases after you submit your IP, contact details, and reason.

Avoid scattershot submissions, don’t file duplicates, and don’t resume full-volume sending until postmaster dashboards, seed tests, and sample inbox placement confirm recovery.

Verification and Ramp-Up

After delisting, send low volume to engaged recipients first, confirm authentication and TLS on the wire, and expand gradually while watching complaint dashboards, latency, and block rates in real time.

Technical Checklist You Can Run Today

Confirm PTR and forward DNS consistency and set the MTA’s greeting to that hostname; test HELO/EHLO from that name across dual stack if you send on IPv4 and IPv6.

Publish or correct SPF with only required includes, deploy DKIM with a rotating selector and strong keys, and enable DMARC with reporting to trace failures and alignment gaps.

Harden submission: require authentication on 587/465, disallow relaying from unknown networks, and rate-limit per user/customer to prevent a single compromise from burning the whole range.

Instrument bounces and complaints with feedback loops where available, automatically suppress hard bounces, and never retry indefinitely into spam traps or retired domains.

IPv6 Considerations

If you send on IPv6, ensure AAAA records, rDNS, and greeting are consistent, and confirm your provider’s blocklist checks understand IPv6 formatting and listings; if you don’t intend to send over IPv6, disable or segregate it so you don’t leak unauthenticated mail on that path.

Key Takeaways

Stop the bleed first, fix authentication and rDNS, remove the actual source of bad traffic, then file targeted delist requests with clear evidence, and resume sending gradually with strong monitoring so you protect delivery and reputation.