Email Blocklist: Check, Fix and Request Removal

One morning, replies dry up and bounce messages start mentioning a list you have never heard of. Your sending IP or domain has landed on an email blocklist, a database that many mail servers consult to decide whether to accept a message. Being listed is stressful, but it is usually fixable, and the listing is a symptom of something specific: compromised accounts, a bad list source, missing DNS records or sending patterns that look like spam. This guide explains what blocklists are, how to check whether you are listed, how to fix the cause and how to request removal properly.

What an email blocklist is

A blocklist, sometimes called a DNSBL or blacklist, is a list of IP addresses or domains associated with unwanted mail. Receiving servers query it during or after the SMTP conversation and may reject, filter or score messages from listed sources.

Different lists track different things. Spamhaus, one of the best-known operators, describes its Combined Spam Sources list as an automatically produced dataset of IP addresses involved in sending low-reputation email (Spamhaus: Combined Spam Sources). Its Domain Blocklist works differently: it is used to check domains that appear in mail headers and message bodies, such as links and contact addresses, after a message has been accepted (Spamhaus: Domain Blocklist). Its Policy Blocklist lists end-user IP ranges that should not send mail directly, and Spamhaus notes that the addresses on it are not necessarily bad.

Knowing which list you are on tells you what kind of problem to look for.

Signs you may be listed

Common signals include:

• Bounce messages that name a blocklist or include a link to a lookup page.
• A sudden drop in delivery to many different organisations at once.
• Rejections that mention reputation or policy rather than unknown users.
• Alerts from a monitoring service that checks your IPs and domains.

Not every delivery problem is a blocklisting. Authentication failures, throttling and spam folder placement produce similar symptoms. Our guide to cold emails going to spam walks through how to tell these apart.

Check your IPs and domains

Spamhaus provides an IP and domain reputation checker, and its blocklist pages point listed senders there for more information and removal steps. Other operators publish their own lookup pages, and several monitoring tools check many lists at once.

Check every address and domain that sends mail for you, not just the main one: your mail server's IP, your sending service's IPs if they are dedicated to you, your sending domain and any domains used in links. If you send through a shared service, a listing of its shared IPs is the provider's issue to resolve, but you should still tell them.

Fix the cause before asking for removal

Requesting removal without fixing the cause rarely works, and repeated listings make removal harder. Investigate first:

• Compromised accounts or servers: look for unusual outbound volume, mail sent outside working hours or messages you did not write.
• List quality: check recent imports, purchased or scraped lists and addresses that have never engaged. Spam traps often sit in old or acquired lists.
• DNS and server setup: Spamhaus's best-practice guidance includes making sure your server's hostname and HELO match and that reverse DNS (a PTR record) is defined and points to the same hostname.
• Outbound port 25: Spamhaus also recommends restricting outbound SMTP traffic so that only your mail server can send directly.
• Links in your mail: if a domain in your messages is listed, check whether it belongs to you, a URL shortener or a third party.

Our guide to spam traps explains how traps end up on lists and how to remove them.

Request removal properly

Once the cause is fixed, follow the operator's own removal process. For the Combined Spam Sources list, Spamhaus says that if you are authoritative for an IP address and believe the issues that caused the listing have been solved, you can request a delisting through its checker. Other operators have their own forms and rules; some remove listings automatically after a period without problems.

When you request removal, be factual. State what you found, what you changed and when. Do not argue that the listing is wrong unless you can show it.

Worked example: a blocklist incident record

The table below is a hypothetical incident record for a company whose mail server IP was listed. It shows the order of work and the details worth recording.

StepWhat we didResultDetectBounces from several recipients named a blocklistListing confirmed via the operator's lookupContainPaused bulk sends from the affected IPUrgent mail moved to the backup sending serviceInvestigateReviewed outbound logs for the past weekFound a compromised mailbox sending unauthorised mailFixReset credentials, enabled multi-factor sign-in, removed the rogue rulesUnauthorised sending stoppedVerify setupChecked PTR record and HELO hostnameBoth correctRequest removalSubmitted request with a summary of the fixListing removed after reviewPreventAdded outbound volume alerts and blocklist monitoringAlerts now go to the IT owner

Keeping a record like this helps if the same problem recurs and makes the next incident faster to handle.

Monitor so you hear first

Set up monitoring that checks your sending IPs and domains against major lists and alerts you when anything changes. Combine it with mailbox provider tools. Microsoft's Smart Network Data Services, for example, gives senders data to understand and improve their reputation at Outlook.com (Outlook.com Smart Network Data Services), and Google Postmaster Tools does the same for Gmail.

Alert on outbound volume as well. A sudden spike in messages from one account or server is often the first sign of a compromise that will lead to a listing.

Reduce the chance of listing again

Most listings trace back to the same few causes. Use double opt-in where it suits your audience, remove addresses that hard bounce, stop mailing contacts who never engage, protect accounts with multi-factor sign-in and keep DNS records correct. Separate bulk campaigns from transactional and personal mail, so that a problem in one stream does not affect the others.

Frequently Asked Questions

What is an email blocklist?

It is a database of IP addresses or domains linked to unwanted mail. Many receiving servers check these lists to decide whether to accept, filter or reject messages.

How do I know if I am on a blocklist?

Look for bounce messages that name a list, then check your IPs and domains with the operator's lookup tool or a monitoring service that checks many lists.

How long does blocklist removal take?

It depends on the operator and the cause. Some remove listings after a request once the issue is fixed; others expire listings automatically after a period without problems.

Can a shared sending service get me listed?

A shared IP can be listed because of another sender. In that case your provider must resolve it, so report it to them and check your own sending as well.

Recover and stay off blocklists

We investigate listings, fix the underlying cause and set up monitoring so that problems are caught early. See our cold outreach and deliverability service, or talk to us about an email blocklist issue.

About the Author

Manav Sharma is the founder of Insyrge and a CRM solutions architect. He works across Zoho, Salesforce, HubSpot, Odoo and more, designing the CRM setups, integrations and automations that keep sales and operations teams working from the same data.