There are a variety of blocklist organizations that will blocklist your domain or IP address. This usually happens when these organizations believe you're sending spam. Yet, only a few of those blocklists are reputable. Most blocklists are insignificant and will have no impact on deliverability.
What is a blocklist?
A blocklist is a list of domains and IP addresses that send spam. These blocklists are updated in real-time by blocklist organizations.
Since the early days of email, mailbox providers needed help protecting their customers from spam. They outsourced much of this work to 3rd parties, which are now known as blocklists. These 3rd parties monitored emails sent worldwide to identify domains and IP addresses that were sending spam. If a blocklist organization caught someone sending spam, they would end up on a blocklist.
Mailbox providers, such as Microsoft, would pay for this blocklist data and block messages from anyone that was on a blocklist. It was a quick way to identify spammers and stop their emails from reaching inboxes.
How blocklists work
Senders end up on a blocklist if they send emails to spam traps that a blocklist is monitoring. Spam traps are email addresses that never opted into email or haven't opted into email for years. Anyone sending to these email addresses is probably not using opt-in data, or their data is very old.
Blocklist organizations often circulate spam traps to different places on the internet. This can include sites that rent and sell lists as well as public pages used by spammers to scrape for lists. A blocklist organization may also monitor old, unused email addresses that were abandoned or shut down. Anyone caught sending to these spam traps are added to blocklists.
What has changed
Major mailbox providers no longer rely on public blocklists to identify spam. Gmail, Microsoft, Yahoo, and others have all developed very sophisticated, proprietary, machine-learning models to identify spammers without having to pay blocklists for this information. The result is that their models are more accurate than ever and harder for spammers to trick.
It also means that blocklists don't have as much impact as they used to. Hundreds of blocklists exist today, many of which are remnants of the 1990s and early 2000s. These blocklists are often abandoned and inaccurate, which is why they are not used by any major email providers - so they do not have any impact on your deliverability.
How to know if you're blocklisted
Most blocklists are public information; anyone can check them. You can use a free tool like Mxtoolbox to see if your IP address or domain appears on public blocklists. Mxtoolbox will show you about 100 blocklists and you may even find yourself listed on some of these. However, 95% of the blocklists you see in a tool like this can usually be ignored.
Note that some blocklists, like Proofpoint (Cloudmark), are not available in Mxtoolbox. This is because of how they're configured. You will need to check with their respective websites directly.
Which blocklists are impactful?
There are only a few blocklists that are cause for alarm and might harm deliverability:
Spamhaus
Spamhaus is the most severe blocklist. If your IP or domain is listed by Spamhaus it means there is a major problem.
Spamhaus has a very sophisticated network of spam traps. These traps are used to identify senders who are sending unsolicited messages to email addresses that never opted in. They have various blocklists with different levels of severity. Being on any Spamhaus blocklist is indicative of a serious problem.
In that case, you should stop immediately from sending emails and carefully review your list, content, and sending practices. You can then ask them to be delisted.
To delist, go to the Spamhaus website and check for a blocklist. If your domain or IP address is on their blocklist, follow the specific instructions on the listing page. Note that you may need to email Spamhaus and open a dialogue with them to be delisted.
Proofpoint (Cloudmark)
Mailbox providers such as Cox, Charter, and Shaw use Proofpoint to identify spam. Proofpoint analyzes messages to identify a spam fingerprint then blocklists an IP associated with those messages. They also have several other data points and can identify many types of spam with accuracy. Proofpoint is the blocklist behind Apple's mailboxes like icloud.com and mac.com. If Proofpoint adds you to their blocklist, your email will fail to deliver to most Apple/Mac/iCloud email addresses.
To delist, complete the Proofpoint (Cloudmark) delisting form and the Proofpoint (Apple) delisting form.
Invaluement
Invaluement is a reputable blocklist that lists IP addresses and domains sending spam, usually with a focus on B2B mail. Invaluement will sometimes list entire IP ranges belonging to email service providers if they allow their customers to send spam. Mailbox providers such as BTinternet in the UK, the domestic provider Mimecast, and gmx/web.de in Germany also use Invaluement to block messages.
To delist, complete the Invaluement delisting form.
Barracuda
Barracuda is a security appliance used by corporations to protect inboxes from inbound spam and phishing. They maintain a reputable blocklist of spammers, but it is has very little impact. Few providers outside of Barracuda will use this blocklist to block messages.
To delist, complete the Barracuda delisting form.
Specific mailbox provider blocklists
There is no public blocklist for major mailbox providers like Gmail, Yahoo, and Microsoft. In rare cases, mailbox providers will block messages they think are spam, but this is not based on a simple blocklist. There are no public places where you can check to see if an IP is blocked at these destinations. More often, they silently place messages in the recipient's spam folder.
With Microsoft, you can see if your IP is blocked via your SNDS portal, and with Gmail, you can see your IP address has a poor reputation via Google Postmaster Tools. However, these are not blocklists; they are indicators that you have a severe reputation problem with Microsoft or Gmail. You will not see these types of issues in Mxtoolbox lookups and you cannot delist them.
Some smaller mailbox providers like Cox and Comcast have blocklists, but these are also not public. If these occur they are usually temporary, only for about 24 hours, and are usually a result of a spike in spam complaints or an issue with a blocklist like Proofpoint or Spamhaus. Because these are specific to a mailbox provider, you will not see these blocklists in Mxtoolbox or any public blocklist checking tool.
What to do if you're on a blocklist
Blocklists are mostly irrelevant for your deliverability unless you find yourself on the blocklists mentioned above. If your domain or IP address appears on any of these blocklists, you should file for delisting using the appropriate forms shared above.
In addition, you'll need to address why you were added to a blocklist. If you don't address the underlying issue, you'll be added back to the blocklist and it will be even harder to remove yourself from it. It's very important to take a hard look at your data collection and sending practices to determine why you were blocklisted and then take action to resolve it.
Here are some common reasons why you may be blocklisted:
You are using old data that needs to be cleaned
We recommend removing contacts who have not opened your emails in the last 6-12 months. Some of these old email addresses may now be recycled spam traps and should be removed from your list. We recommend using the Engagement Management tool to accomplish this.
You are sending to emails that have not opted in
If you are sending emails to contacts who did not opt into your content or brand, you need to remove these contacts or stop sending them content they do not wish to receive. For example, sending emails to purchased, traded, or appended lists will cause you to be blocklisted.
Your forms were attacked by a spam bot
If your forms were attacked by a spam bot, then your list may be full of spam traps or email addresses of real people who did not opt in to receive emails from you. If you suspect this is happening, we recommend adding captcha to your forms and using a double opt-in process.
Your email addresses may contain typos
If you're manually entering email addresses into ActiveCampaign, you may accidentally end up with misspelled email addresses that are actually spam traps. Try to use a subscription form on your website with double opt-in as much as possible to combat this.
If you follow the fundamentals and send engaging content to clean, opt-in lists, you should not find yourself on any blocklist.