How does an IP address being on a blocklist affect domain reputation and email open rates?
Matthew Whittaker
Co-founder & CTO, Suped
Published 10 Jul 2025
Updated 17 Aug 2025
11 min read
Email deliverability is a complex ecosystem where various factors influence whether your messages land in the inbox or the spam folder. Among the most critical of these factors is your sender reputation, which is broadly divided into IP reputation and domain reputation. I've seen firsthand how a single issue, like an IP address ending up on a blocklist (or blacklist), can have far-reaching consequences, even impacting something as fundamental as email open rates.
The immediate effect of a blocklisted IP is a severe disruption to email delivery. Mailbox providers and internet service providers (ISPs) rely heavily on these lists to identify and block (or filter) unwanted mail. If your sending IP is on one, it's a clear signal to these providers that your mail might be spam or originate from a compromised source, leading to high rejection rates or direct delivery to the spam folder. This can be particularly frustrating when you are trying to understand why your emails are no longer reaching their intended recipients.
This situation becomes even more convoluted when dealing with shared IP addresses, which are common with many email service providers. If one user on a shared IP abuses it or becomes compromised, the entire IP can get blacklisted, affecting all other legitimate senders using that same IP. Even if your sending practices are impeccable, you could suffer the consequences of someone else's poor behavior. This is why it is so important to check on these issues, even if you are not directly at fault.
This article will explore the intricate relationship between IP blocklists, domain reputation, and the tangible impact on your email open rates. We will also look at how to diagnose and address these issues, ensuring your email program remains healthy and effective.
An IP blocklist (often interchangeably called a blacklist) is essentially a database of IP addresses that have been identified as sources of spam or other malicious email activity. These lists are maintained by various organizations, such as Spamhaus, and are used by email providers globally to filter incoming mail. When your sending IP address appears on one of these lists, it signals to receiving mail servers that emails from that IP should be treated with suspicion, if not outright blocked. I've often seen this lead to significantly reduced email deliverability rates, sometimes by as much as 95%, as highlighted by GetMailbird.
The primary reasons an IP address gets blocklisted usually stem from poor sending practices. This could include sending unsolicited emails (spam), having a high rate of spam complaints from recipients, sending to invalid or old email addresses that turn into spam traps, or even being compromised by malware that sends spam without your knowledge. These actions erode the IP's reputation, making it appear untrustworthy to the wider email ecosystem. You can learn more about this in our guide to how email blacklists actually work.
The immediate impact of an IP blocklisting is straightforward: emails sent from that IP are less likely to reach the inbox. They might be bounced, quarantined, or sent directly to the spam folder, bypassing the primary inbox entirely. This directly translates to lower email open rates because your messages simply aren't visible to your intended audience. This is particularly true for major providers like Gmail and Yahoo Mail, which heavily rely on these blocklists.
IP reputation versus domain reputation
While an IP address being on a blacklist has a direct impact on deliverability, it's also important to understand its connection to your domain reputation. Your overall sender reputation is a combination of both your IP reputation and your domain reputation. Think of it this way: the IP is the street your house (domain) is on. If the street has a bad reputation, it can reflect poorly on your house, even if your house is well-maintained.
Even if your domain isn't directly blacklisted, a blocklisted IP sending mail on its behalf can still degrade your domain's standing. Mailbox providers observe sending patterns from both the IP and the domain. A consistent pattern of emails from a suspicious IP associated with your domain can lead them to believe your domain is also engaging in questionable practices. This is particularly relevant when you're using shared IP pools, as one bad actor can taint the reputation for everyone on that pool. You can learn more about this interplay in our discussion on IP versus domain reputation.
The impact on domain reputation might not be as immediate or severe as a direct domain blocklisting, but it is cumulative. If your emails consistently fail to reach the inbox due to a poor IP reputation, recipients won't engage with them. Low engagement, coupled with potential spam complaints stemming from non-delivery, eventually sends negative signals about your domain, even if it's technically 'clean' on its own. This creates a vicious cycle that can be difficult to break.
IP reputation
Measures the trustworthiness of the specific server IP address sending emails. Influenced by direct sending behavior from that IP, such as spam complaints, bounce rates, and volume consistency.
Shared IPs: Reputation can be affected by the sending practices of others sharing the same IP.
Warm-up: New IPs require a gradual sending increase to build a positive reputation.
Domain reputation
Measures the trustworthiness of your domain (e.g., example.com) itself. Influenced by subscriber engagement, content quality, and consistent authentication (SPF, DKIM, DMARC).
Brand impact: Directly tied to brand recognition and trust with recipients.
Persistence: Reputation builds over time and is harder to damage or repair quickly.
Impact on email deliverability and open rates
The most direct consequence of a blocklisted IP is the severe hit to your email deliverability. When an IP is blacklisted, it's flagged as a source of undesirable email. This means that a significant portion, if not all, of your emails sent from that IP will be filtered out by receiving mail servers. They might be rejected outright, sent to the spam or junk folder, or simply disappear without a trace. This effectively prevents your messages from reaching the intended inboxes, regardless of the quality of your content or the legitimacy of your sending practices.
This poor deliverability inevitably leads to a sharp decline in email open rates. If your emails aren't landing in the primary inbox, recipients won't see them, and therefore can't open them. Even if some messages trickle into the spam folder, the likelihood of them being opened is significantly lower. I've witnessed situations where open rates plummeted from healthy percentages to single digits, or even near zero, directly correlating with an IP blocklisting. This problem is compounded by providers like Google and Yahoo implementing stricter sender requirements.
Beyond the immediate numbers, a persistent issue with IP blacklisting can severely damage your overall sender reputation. Mailbox providers build a profile of your sending behavior over time. If they consistently see your emails associated with a blocklisted IP, even if your domain isn't explicitly listed, it erodes their trust in your brand as a legitimate sender. This long-term damage can be much harder and slower to repair than simply getting an IP delisted.
Diagnosing and resolving blocklist issues
The first step in addressing a blocklisted IP is to confirm the listing and its source. There are many online tools that allow you to check if your IP address or domain is listed on various blacklists. Once you identify the specific blacklist (or blocklist), visit their website to understand the reason for the listing and their specific delisting process. This initial diagnosis is crucial before taking any corrective actions.
If you're using a shared IP address provided by an Email Service Provider (ESP), the responsibility for delisting often falls on them. You'll need to contact your ESP's support team and provide them with the details of the blocklisting. They typically have established procedures and relationships with blacklist operators to facilitate the removal process. However, if it's your dedicated IP, you'll need to follow the blacklist's instructions directly. The process for resolving blocklisting issues can vary depending on the specific list.
Crucially, simply getting delisted isn't enough. You must identify and rectify the underlying cause of the blacklisting. This might involve cleaning your email lists, implementing stricter spam complaint monitoring, ensuring your sending infrastructure isn't compromised, or improving your email authentication protocols like SPF, DKIM, and DMARC. Without addressing the root cause, your IP (or a new one) is likely to end up on a blacklist again.
Steps for delisting and recovery
Check status: Use a blocklist checker to confirm the IP address is indeed listed and on which specific blacklist.
Identify the cause: Investigate why the IP was listed. Common reasons include spam complaints, sending to spam traps, or malware activity.
Remediate issues: Fix the problem. This might involve cleaning your email list, adjusting sending volume, or securing compromised systems.
Request removal: Follow the specific delisting instructions provided by the blacklist operator. If using a shared IP, contact your ESP.
Preventing an IP from being blacklisted is far more effective than trying to recover from one. Maintaining a healthy sending reputation requires a proactive and consistent approach. This starts with diligent list hygiene. Regularly clean your email lists to remove inactive subscribers, hard bounces, and any addresses that have converted into spam traps. Sending to a clean, engaged list is fundamental to good deliverability.
Consistent sending volume and careful content creation are also key. Avoid sudden, large spikes in email volume, as this can trigger spam filters. Ensure your email content is relevant, engaging, and doesn't resemble spam. This includes avoiding excessive capitalization, too many images, or suspicious links. Also, make sure your email authentication records (SPF, DKIM, DMARC) are correctly configured and enforced, as these are critical for proving your legitimacy.
Finally, proactive monitoring is indispensable. Regularly check your IP and domain against major blacklists using a blocklist checker. Pay attention to bounce rates and spam complaint rates reported by your ESP or through tools like Google Postmaster Tools. Early detection of issues can prevent them from escalating into full-blown blocklistings and safeguard your open rates.
Views from the trenches
Best practices
Actively monitor all IP addresses associated with your domain's email sending, not just those you directly manage.
Regularly review your email sending practices for any deviations that could negatively impact your reputation, such as sudden volume spikes or content changes.
Ensure proper email authentication, including SPF, DKIM, and DMARC, is consistently configured and aligned with your sending sources.
Segment your email lists and tailor content to ensure high engagement and low spam complaint rates across different recipient groups.
Maintain robust bounce management to promptly remove invalid and non-existent email addresses from your sending lists.
Common pitfalls
Overlooking an IP blocklist because your domain itself isn't listed, failing to recognize the indirect impact on domain reputation and deliverability.
Assuming an old IP blocklisting from a shared provider is irrelevant, especially if the IP is still associated with your domain's sending.
Not understanding the actual sender IP address for emails, particularly when using third-party ESPs or shared IP pools.
Neglecting to investigate the root cause of a blocklisting, which can lead to recurring issues even after a temporary delisting.
Underestimating the cumulative effect of low engagement and spam complaints on overall sender reputation, which IP blocklists exacerbate.
Expert tips
If you're using a shared IP pool, closely monitor your email service provider's overall reputation, as their practices impact your deliverability.
Implement a DMARC policy with reporting to gain visibility into your email authentication results and identify unauthorized sending sources.
For critical email streams, consider using dedicated IP addresses to have more control over your sending reputation.
Analyze your email engagement metrics closely, as declining open rates can be an early indicator of underlying deliverability issues.
Work with your ESP to understand how their IP management practices affect your sender reputation and discuss options for improved deliverability.
Marketer view
Marketer from Email Geeks says they had a client with declining open rates and found one of their IPs on Spamhaus ZEN via MXtoolbox, even though the domain wasn't listed. They were confused since they used HubSpot with shared IPs and Microsoft as their email service provider.
2024-12-05 - Email Geeks
Expert view
Expert from Email Geeks says the IP in question was a Microsoft address, specifically generic Azure space without reverse DNS. It was unlikely to be a HubSpot sending IP and wasn't listed in HubSpot's SPF record ranges.
2024-12-05 - Email Geeks
Maintaining deliverability and open rates
An IP address being on a blocklist (or blacklist) has a profound and undeniable impact on both your domain reputation and, consequently, your email open rates. While IP reputation and domain reputation are distinct, they are deeply interconnected, forming the cornerstone of your overall sender reputation. A poor IP reputation, whether due to your own sending practices or those of others on a shared IP, will directly impede your emails from reaching the inbox, leading to a significant drop in engagement.
To protect your email program, it's essential to proactively monitor your sending IP(s) and domain for any blocklistings. Rapid identification and remediation of issues, coupled with consistent adherence to email best practices, are crucial. This includes maintaining clean lists, ensuring proper email authentication (SPF, DKIM, DMARC), and fostering positive recipient engagement.
Ultimately, a healthy sender reputation is not a static achievement but an ongoing commitment. By understanding the intricate dynamics of IP and domain reputation, and actively managing your sending practices, you can ensure your emails consistently land in the inbox, maintaining strong open rates and maximizing the effectiveness of your email communications.
Spamhaus
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Cisco
NoSolicitado
URIBL
abuse.ro
ALPHANET
Anonmails
Ascams
BLOCKEDSERVERS
Calivent Networks
EFnet
JustSpam
Kempt.net
NordSpam
RV-SOFT Technology
Scientific Spam
Spamikaze
SpamRATS
SPFBL
Suomispam
System 5 Hosting
Team Cymru
Validity
www.blocklist.de Fail2Ban-Reporting Service
ZapBL
2stepback.dk
Fayntic Services
ORB UK
technoirc.org
TechTheft
