Email Blocklist as a concept is often forgotten about by those outside of deliverability. If your emails have started landing in spam, or your open rates have dropped without an obvious explanation, one of the first things to check is whether your domain or sending IP is on an email blocklist.
It is not always the cause. But it is one of the quickest things to rule out, and finding out is straightforward.
This post explains what blocklists are, how domains end up on them, which ones actually matter, how to check, and what to do if you find you are listed.
What an email blocklist actually is
A blocklist, sometimes called a DNSBL (Domain Name System Blocklist), is a database of domains and IP addresses that have been flagged for sending spam or exhibiting suspicious sending behaviour.
Inbox providers and spam filters consult these lists when deciding how to handle incoming email. Being listed does not automatically mean your emails will be blocked, but it does work against you. Depending on which blocklist you are on and how the receiving mail system is configured, the effect can range from emails being filtered to spam to being rejected entirely.
There are hundreds of blocklists in operation. Most of them are irrelevant. A small number carry genuine weight with inbox providers and spam filters, and those are the ones worth paying attention to.
Not all blocklists are equal
Being listed on an obscure blocklist with limited adoption is unlikely to affect your deliverability in any meaningful way. The blocklist ecosystem includes a long tail of lists that few inbox providers actually consult.
The more important point: some blocklists charge a fee to have your domain or IP removed. That is a red flag. Reputable blocklists do not charge for delisting. If you find yourself listed on one that is asking for payment, it is almost certainly not a list that matters. Do not pay it.
The blocklists that carry real weight are operated by organisations with established credibility in the anti-spam community. They have clear listing criteria, transparent delisting processes, and genuine adoption among major inbox providers.
The blocklists that actually matter
Spamhaus is the most influential blocklist operator in the world. Their lists, including the Spamhaus Block List (SBL), the Exploits Block List (XBL), and the Policy Block List (PBL), are widely used by inbox providers globally. A Spamhaus listing is serious and needs to be addressed promptly. Their delisting process is free and documented clearly on their website: www.spamhaus.org
Barracuda Reputation Block List (BRBL) is maintained by Barracuda Networks and is used heavily in enterprise email environments, including many UK businesses running Barracuda email security appliances. Being listed here can cause significant delivery failures to corporate recipients. Check and request delisting at: www.barracudacentral.org/lookups
Invaluement and SURBL focus on domains found in spam message content rather than sending infrastructure. If your domain is appearing in spam sent by others, or has been used in phishing campaigns, you may find yourself listed here. Check Invaluement at www.invaluement.com/lookup and SURBL at www.surbl.org/surbl-analysis
SpamCop is one of the older blocklist operators and still has meaningful adoption, particularly with ISPs. It operates on a relatively short listing window, so listings often expire on their own, but it is worth knowing about. You can check your status at www.spamcop.net
For UK senders specifically, some blocklists carry higher weighting with European mail providers. If a significant portion of your list is in the EU, this is relevant to monitor.
How domains end up on blocklists
Blocklisting is usually the result of sending behaviour that has triggered spam filters or complaint thresholds. Common causes include high spam complaint rates, sending to spam trap addresses, sudden volume spikes from a domain with limited sending history, and authentication failures.
Spam traps are worth understanding specifically. They are email addresses maintained by blocklist operators and inbox providers to catch senders with poor list hygiene. Some were once real addresses that have been recycled. Others were never real at all, and any email sent to them is by definition unsolicited. If your list has grown without proper hygiene practices, or includes addresses from third-party sources, you may be hitting traps without knowing it.
It is also possible to end up on a blocklist through no fault of your own. If you are on a shared IP address, another sender using that same IP can affect your deliverability. This is one of the arguments for dedicated IP addresses at higher sending volumes, though it comes with its own requirements around warming and maintenance.
How to tell if a blocklist is affecting you
One of the clearest early signals is your soft bounce data. When a receiving mail server rejects an email because your IP or domain is on a blocklist, it will typically return a bounce code and a reason in the bounce message. Codes in the 5xx range indicate a permanent rejection, but blocklist-related rejections often appear as 4xx soft bounces initially, with the reason citing a specific blocklist by name.
If your ESP provides access to bounce reason data, it is worth checking it regularly. A pattern of soft bounces citing the same blocklist across multiple sends is a clear signal that something needs addressing. Many senders overlook this data, which is why blocklist issues often go undetected until they have been affecting deliverability for some time.
How to check if you are listed
If you want a quick starting point, Digistrat's free email deliverability health check lets you check your domain and IP reputation alongside your authentication setup, with no sign-up required. It is a good first step if you want a broad picture of where things stand before digging into individual tools.
For more granular blocklist checking, MXToolbox is the most widely used option. Enter your domain or IP and it will check against a broad range of lists and return a clear pass or fail for each. Multirbl.valli.org covers a wider range of lists if you want more comprehensive results. Google Postmaster Tools shows your domain reputation with Gmail specifically, which is worth monitoring as a baseline regardless of blocklist status.
If you are using a major ESP, they may also provide deliverability reporting that flags blocklisting issues. Some include inbox placement monitoring tools that give you a clearer picture of where your emails are actually landing.
Checking once is useful. Checking regularly is better. Blocklist status can change, and catching a listing early is considerably easier than dealing with it after it has been affecting deliverability for weeks.
What to do if you are listed
Finding yourself on a blocklist is not a disaster, but it does need to be dealt with promptly.
The first step is understanding why. A listing without understanding the cause will come back, because the underlying behaviour that triggered it has not changed. Look at your complaint rates, your bounce data, your list sources, and your sending patterns.
Once you understand the cause and have addressed it, most reputable blocklists have a delisting request process. Spamhaus and Barracuda both have online forms. Some delisting is automatic after a period of clean sending. Others require a manual review. All of the blocklists worth caring about offer this for free.
Do not request delisting before the underlying issue is fixed. Repeat listings are harder to resolve and can result in longer or permanent blocks on some lists.
If you are on multiple lists simultaneously, or listed on a high-impact list like Spamhaus, that is a signal the issues run deeper than a single incident. It is worth getting a proper assessment of your overall sending infrastructure at that point, rather than just addressing the listing in isolation.
Start with a free check
If you are not sure where your domain stands, the free email deliverability health check is the quickest way to find out. It checks your authentication setup, your domain reputation and your IP reputation in one place, with no account needed.
Run a free deliverability health check
If the results raise questions, or you are already seeing signs that deliverability is affecting your email performance, a Deliverability Audit will give you the full picture and a clear plan for what to fix.
