Why is my IP listed on blocklists despite sending legitimate email?
Michael Ko
Co-founder & CEO, Suped
Published 19 Jul 2025
Updated 17 Aug 2025
8 min read
It can be incredibly frustrating to find your IP address on an email blocklist (or blacklist), especially when you're confident you're only sending legitimate emails. You diligently craft your messages, target engaged recipients, and follow best practices, yet suddenly, your emails are bouncing, and you're flagged as a sender of unwanted mail. This situation often arises due to factors beyond your immediate control or a misunderstanding of how blocklists operate.
The perception that only malicious senders get listed is a common misconception. Many legitimate organizations, from small businesses to large enterprises and non-profits, encounter this challenge. The reality is that the mechanisms used to combat spam are complex and sometimes cast a wide net, catching innocent senders in the process.
Understanding the common reasons behind these listings, even for what you consider legitimate email, is the first step toward diagnosis and resolution. It involves looking at your sending infrastructure, list hygiene, and recipient engagement.
One of the most frequent causes of unexpected IP blacklistings is the use of a shared IP address. If you're sending emails through a shared email service provider or web host, your IP address is also used by numerous other senders. The actions of just one bad actor on that shared IP can negatively impact everyone else using it, even if your sending practices are impeccable. This means an IP could be blocklisted due to someone else's spam, malware, or compromised account.
Internet Service Providers (ISPs) and email providers often assign dynamic IP addresses to residential and some business connections. These IPs are frequently recycled, meaning an IP you're currently using might have been previously used by a spammer. When an IP has a history of abuse, it can carry a poor reputation, leading to immediate blocklistings even if your current usage is entirely legitimate. In these scenarios, the IP is often listed on policy blocklists (PBLs) like Spamhaus PBL, which are designed to list IPs that should not be sending email directly to the internet.
The nature of shared IP environments makes them prone to these issues. While convenient for low-volume senders, they introduce a dependency on the sending habits of others on the same server. This is a common challenge for those without the volume to justify a dedicated IP. Sometimes, a shared webhost IP gets blocklisted, impacting legitimate email sending.
Dedicated versus shared IP addresses
Shared IP addresses
Reputation volatility: Your sender reputation is tied to others using the same IP, making it vulnerable to their poor sending practices.
Cost-effectiveness: Often more affordable and suitable for lower email volumes without significant deliverability needs.
Limited control: Less control over your IP's sending history and reputation building.
Dedicated IP addresses
Reputation control: Your sender reputation is solely based on your sending habits, offering more stability.
Higher cost: Generally more expensive and requires consistent high volume sending to maintain reputation.
Warming period: Requires a gradual increase in sending volume to build a positive reputation.
Recipient complaints and spam traps
Even with perfectly legitimate emails, recipient actions can lead to your IP being blocklisted. If a significant number of your recipients mark your emails as spam, even if they initially opted in, it signals to mailbox providers that your mail is unwanted. These spam complaints are a major factor for blocklists. Mailbox providers (like Google and Microsoft) pay close attention to user feedback, and high complaint rates can swiftly lead to an IP or domain blocklisting.
Another sneaky culprit is spam traps. These are email addresses specifically set up by blocklist operators and ISPs to catch spammers. Sending to a spam trap, even accidentally, is a strong indicator of a questionable list. There are different types, from pristine (never used for anything but a trap) to recycled (old, abandoned addresses). Hitting a spam trap can instantly land your IP on a severe blocklist (or blacklist), regardless of your content or sender reputation.
The key here is list hygiene. Regularly cleaning your email list to remove inactive or invalid addresses is crucial. This proactive approach minimizes bounces, reduces the chance of hitting spam traps, and keeps your complaint rates low, which are essential for maintaining a good sender reputation. Learn how to get off an email blocklist and stay off, by avoiding these common pitfalls.
Technical misconfigurations
Even if your email content is perfectly legitimate, underlying technical issues can lead to blocklisting. This includes misconfigured SPF, DKIM, or DMARC records. These email authentication protocols help prove that your emails are genuinely from your domain and haven't been spoofed. Failure to implement these correctly can lead to emails being flagged as suspicious by receiving mail servers, even if they are legitimate.
Another technical aspect is your mail server's configuration. An open relay, a compromised server, or even incorrect reverse DNS (PTR record) can make your server appear suspicious. Cybercriminals actively seek out vulnerable mail servers to send spam, and if your server is compromised, it could be sending spam without your knowledge, leading to a blocklisting. Malware on a network machine, as suggested by Quora discussions, can also be a cause.
It’s also important to consider the content and formatting of your emails. While you believe your content is legitimate, certain elements might trigger spam filters. This includes excessive use of certain keywords, broken HTML, or too many images relative to text. Even if your intent is good, the technical presentation can inadvertently lead to emails being flagged as spam.
What to do when your IP is blocklisted (or blacklisted)
If you find your IP blocklisted, the first step is to accurately identify which blocklist(s) it's on. Bounce messages often provide clues. Many blocklists allow you to check an IP or domain directly on their website. It's common for an IP to be listed on some less impactful blocklists without affecting most of your mail flow. However, major blocklists (or blacklists) like Spamhaus or Cloudmark CSI-Global can severely impact your deliverability.
Once identified, the next step is to understand the reason for the listing. Blocklist websites usually provide details on why an IP was added and what steps are required for delisting. This might involve cleaning up your list, fixing authentication records, or addressing a compromised system. Prompt action is critical, as prolonged listings can cause significant damage to your sender reputation and email program.
For ongoing protection, implement a robust blocklist monitoring strategy. This ensures you're immediately alerted to new listings, allowing you to take swift action before deliverability is severely impacted. Prevention is always better than cure in the world of email. Use a tool like MultiRBL to check your IP against multiple blocklists.
Immediate steps if you're blocklisted
Action plan
Check all relevant blocklists: Don't rely on just one checker; use several to get a comprehensive view. Your IP might be listed on some blocklists but not others. Bounce messages often indicate the specific blocklist.
Identify the cause: Look for high spam complaints, recent list imports, or any unusual sending activity. Check for malware on your network or compromised accounts.
Address the root issue: Before requesting delisting, fix the underlying problem that led to the listing. This could involve segmenting your lists or improving engagement.
Request delisting: Follow the specific instructions provided by each blocklist. Be honest about the issue and the steps taken to resolve it.
Monitor your reputation: Continuously track your IP and domain reputation to prevent future listings and ensure consistent deliverability.
Conclusion
In the world of email, even the most diligent senders can face the challenge of an IP blocklisting. It’s not always an indictment of malicious intent but rather a reflection of the complex ecosystem of anti-spam measures. By understanding the common culprits, from shared IP issues to recipient complaints and technical misconfigurations, you can better equip yourself to diagnose and resolve these frustrating situations.
Proactive measures are your best defense. Regularly auditing your sending practices, maintaining a clean email list, and ensuring your email authentication (like SPF, DKIM, and DMARC) is correctly configured can significantly reduce your risk. When a listing does occur, swift action based on accurate information is key to minimizing disruption and protecting your sender reputation. With the right approach, you can navigate these challenges and ensure your legitimate emails continue to reach their intended inboxes.
Views from the trenches
Best practices
Regularly monitor your IP and domain reputation across multiple blocklists to catch issues early and prevent widespread impact.
Implement and verify SPF, DKIM, and DMARC records to ensure email authentication passes consistently and builds sender trust.
Maintain a clean email list by regularly removing inactive subscribers, hard bounces, and known spam traps to reduce complaint rates.
Common pitfalls
Over-relying on a single blocklist checker, as some blocklists are private or more niche, leading to incomplete diagnosis.
Ignoring bounce messages, which often contain specific information about the blocklist responsible for the email rejection.
Not addressing the root cause of a listing before requesting delisting, which can lead to rapid re-listing.
Expert tips
If using a shared IP, communicate with your email service provider about the blocklist listing and their strategy for managing shared IP reputation.
Review your email sending logs for unusual activity, such as spikes in volume or sending to unknown recipients, which may indicate compromise.
Educate your recipients on how to whitelist your emails or move them from spam to inbox, reducing future complaint signals.
Marketer view
Marketer from Email Geeks says they experienced an unusually large number of block bounces for their nonprofit newsletter, despite the content being vetted.
February 23, 2024 - Email Geeks
Marketer view
Marketer from Email Geeks says their bounce messages indicated their shared IP was listed on gbudb.com/truncate or spamrl.com, but MX Toolbox showed it as not blacklisted.
Spamhaus
0Spam
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
Google and 
Microsoft) pay close attention to user feedback, and high complaint rates can swiftly lead to an IP or domain blocklisting.
