Why is my IP address blacklisted when sending email, and how can I fix it?
Matthew Whittaker
Co-founder & CTO, Suped
Published 12 Jul 2025
Updated 16 Aug 2025
10 min read
Suddenly, your emails aren't reaching their destination. You're getting bounce messages, and your carefully crafted campaigns are failing to land in inboxes. The culprit might be a blacklisted (or blocklisted) IP address. This can be a frustrating experience, especially when you're sure you're sending legitimate emails.
An IP address blacklist is essentially a public list of IP addresses that have been flagged for sending spam or other malicious content. Internet Service Providers (ISPs) and email providers use these lists to filter incoming mail, protecting their users from unwanted messages. If your IP ends up on one of these lists, your emails are likely to be rejected or sent straight to the spam folder, severely impacting your deliverability.
Understanding why your IP address was blacklisted is the first step toward resolving the issue and preventing it from happening again. It's a common challenge in the email world, but with the right approach, it's fixable.
IP blacklists, also known as blocklists, are databases of IP addresses with poor reputations. These lists are maintained by various organizations and are used by email providers worldwide to identify and block (or filter) suspicious email traffic. When your IP address appears on such a list, it signals to receiving mail servers that emails originating from your IP might be spam or malicious.
The impact of an IP blacklist is significant. It can lead to emails bouncing back, being diverted to spam folders, or even being completely rejected by email services. This not only affects your ability to communicate effectively but also damages your sender reputation, making it harder for your legitimate emails to reach the inbox in the future.
There are hundreds of blacklists out there, some public and some private (internal to specific email providers like Gmail or Outlook). While some lists are highly influential, others might only impact a small segment of your audience. The key is to identify which lists you're on and why.
Common reasons for IP blocklisting
Several factors can lead to an IP address being blacklisted, even if you believe you're sending legitimate emails. Identifying the root cause is crucial for effective remediation.
Spam complaints: One of the most common reasons. If recipients mark your emails as spam, it quickly signals to ISPs that your sending practices are problematic. Even a low complaint rate can lead to blacklisting.
High bounce rates: Sending to invalid or non-existent email addresses results in hard bounces. High hard bounce rates suggest poor list hygiene and can trigger blacklist listings.
Malware or compromised server: If your server or any machine behind your network address translation (NAT) is infected with malware, it could be secretly sending out spam emails without your knowledge, leading to a blacklist (or blocklist) entry.
Poor sending practices: Sending a sudden, large volume of emails, using generic or suspicious content, or not warming up new IPs can all contribute to a poor sender reputation.
Misconfigured DNS records: Missing or incorrect SPF, DKIM, or DMARC records can make your emails appear less legitimate, even if they're not spam. Some blacklists also check for proper PTR records and HELO/EHLO commands.
Shared IP issues: If you're using a shared IP address, the sending habits of other users on that same IP can affect your deliverability. Their poor practices can get the shared IP blacklisted, even if your sending is impeccable.
In some cases, especially with dynamic residential IP addresses or IPs behind a NAT (Network Address Translation) that also serve other machines like Windows desktops, you might find your IP blocklisted due to traffic originating from other devices. It's crucial to thoroughly investigate your network for any anomalies if this is a recurring issue.
How to check if your IP is blacklisted
The first step in addressing an IP blacklist (or blocklist) is to confirm your IP's status. Many online tools allow you to check if your IP address is listed on major blacklists.
Online blacklist checkers can scan your IP against multiple popular blocklists simultaneously. You simply enter your IP address, and the tool provides a report indicating any listings. Some reputable lists include Spamhaus, UCEPROTECT, and SORBS. Remember that each list has its own criteria and removal process. You can also use a blocklist checker.
If your IP is listed, the report will usually indicate which specific blocklist(s) have flagged it. This information is crucial because each blacklist operator has a different process for delisting. Some removals are automatic after a period of clean sending, while others require a manual request and often a detailed explanation of the steps taken to resolve the underlying issue.
How to remove your IP from a blacklist
Once you've confirmed your IP address is blacklisted, it's time to take action. This typically involves a three-step process: identifying the root cause, fixing it, and then requesting removal.
Identify and fix the root cause
Before you even think about delisting, you must understand and fix why you were blacklisted in the first place. Failure to do so will almost certainly result in your IP being relisted quickly. If it was due to a compromised server, you need to scan your systems for malware and secure any vulnerabilities. If it's related to email content or sending volume, adjust your campaigns and list management practices. For issues with HELO/EHLO or bare IP addresses, ensure your mail server is configured correctly to use a fully qualified domain name (FQDN).
Request delisting
Each blacklist (or blocklist) has its own delisting procedure, usually found on their website. This often involves filling out a form, providing your IP address, and explaining the steps you've taken to resolve the issue. Be honest and thorough in your explanation.
For specific providers, like Microsoft (Outlook/Hotmail), you might need to use their dedicated delisting portal or email delist@microsoft.com with your bounce message and IP address. Similarly, Gmail has specific forms and postmaster tools for managing sender reputation. Delisting can take anywhere from a few hours to several days, depending on the blacklist and the severity of the issue.
Monitor your IP and deliverability
After delisting, continuous monitoring is essential. Regularly check your IP address on blacklists (or blocklists) and pay close attention to your email deliverability metrics, such as open rates, click-through rates, and bounce rates. Low engagement or high complaint rates can signal that your sending practices still need improvement, potentially leading to future listings.
Preventing future blacklistings
Preventing future IP blacklistings is all about maintaining a healthy sender reputation and adhering to email sending best practices.
Maintain clean email lists: Regularly remove inactive or invalid email addresses to minimize bounce rates and avoid spam traps. Implement a double opt-in process for new subscribers.
Implement strong authentication: Ensure your SPF, DKIM, and DMARC records are correctly configured. These authentication protocols prove that your emails are legitimate and prevent spoofing. A simple guide to email authentication can help.
Monitor engagement and complaints: Keep an eye on your spam complaint rates and subscriber engagement. High complaint rates or low engagement can be early warning signs.
Warm up new IP addresses: If you acquire a new IP address, gradually increase your sending volume over time. This helps build a positive sending reputation with ISPs.
Review your content: Avoid spammy keywords, excessive images, or poor formatting that can trigger spam filters. Ensure your emails provide value to recipients.
Secure your sending infrastructure: Regularly audit your servers and systems for vulnerabilities, malware, or open relays that could be exploited by malicious actors to send spam.
By proactively managing your email program and staying vigilant about your sending reputation, you can significantly reduce the risk of your IP address appearing on an email blacklist or blocklist.
Diving deeper into technical fixes
Successfully getting your IP delisted and maintaining a good sender reputation often requires a deep dive into technical configurations and consistent monitoring. It's not just about removing the listing, but understanding and fixing the underlying issues.
One common issue is sending emails from an IP behind a NAT, especially if that NAT also serves general-purpose machines like Windows desktops. If any machine behind the NAT becomes compromised, it can start spewing spam, leading to the public-facing IP address getting blacklisted (or blocklisted). The most reliable solution for this scenario is to use a dedicated mail server with its own real IP address, not one behind a shared NAT.
Another area to investigate is your HELO/EHLO commands. If a blacklist (or blocklist) suggests non-compliance, even if you think you're sending an FQDN, it's worth double-checking. Sometimes, a subtle misconfiguration can lead to your mail server presenting a bare IP address instead of the expected domain name during the SMTP transaction, which can lead to a blocklist entry.
Views from the trenches
Best practices
Always maintain robust security practices to prevent malware infections on your sending servers or any devices behind shared NATs.
Implement strong email authentication (SPF, DKIM, DMARC) and regularly verify their correct configuration for all sending domains.
Segment your email lists and send targeted content to highly engaged subscribers to reduce spam complaints and improve reputation.
Use dedicated IP addresses for high-volume or critical email sending to isolate your sending reputation from other users.
Common pitfalls
Ignoring early warnings from email service providers about deliverability issues or increased spam complaints.
Failing to thoroughly investigate and fix the root cause of a blacklist listing before requesting removal, leading to quick relisting.
Sending emails to old, unengaged, or purchased lists, which can trigger spam traps and high bounce rates.
Not implementing or incorrectly configuring email authentication protocols, making your emails appear suspicious.
Expert tips
Regularly perform packet sniffing on your outbound mail traffic to ensure that HELO/EHLO commands are correctly sending FQDNs and not bare IP addresses.
If using VMs behind a NAT, scan all virtual machines for malware or any unauthorized email-sending processes.
For recurring blacklist issues, consider whether your network architecture (e.g., sending mail from behind a general-purpose NAT) is fundamentally conducive to email deliverability.
Always document the steps taken to remediate the underlying issue when submitting a delisting request to a blacklist operator.
Marketer view
A marketer from Email Geeks says they were concerned about multiple IPs in a series getting blacklisted, even after checking for HELO/EHLO compliance and exploits, indicating a deeper network issue.
2021-07-27 - Email Geeks
Expert view
An expert from Email Geeks says that if blacklisting is persistent, it's highly likely that something is connecting from that IP with a bare IP address, implying an infection or open relay is emitting spam.
2021-07-27 - Email Geeks
Keeping your email reputation clean
An IP address blacklisting can be a major roadblock to your email deliverability, but it's a solvable problem. The key is to act swiftly, thoroughly investigate the cause, and implement lasting solutions rather than quick fixes. Understanding the common reasons for blacklisting, knowing how to check your IP status, and following a clear remediation process are crucial steps.
Long-term success in email deliverability hinges on proactive reputation management. This includes maintaining clean mailing lists, sending valuable content, and ensuring your email authentication (like DMARC) is robust. By doing so, you'll not only resolve current blacklist issues but also build a strong foundation for reliable email communication, keeping your messages consistently landing in the inbox.
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