Why is my IP repeatedly blocklisted by Spamhaus XBL?
Michael Ko
Co-founder & CEO, Suped
Published 12 Aug 2025
Updated 16 Aug 2025
8 min read
Dealing with an IP address repeatedly appearing on the Spamhaus XBL (Exploits Blocklist) is one of the most frustrating challenges in email deliverability. I've seen many administrators struggle with this, especially when they diligently delist their IP only to find it back on the blacklist shortly after. The bounce messages often point to a specific issue: making SMTP connections with HELO values that use a bare IP address.
This specific reason for listing is a strong indicator of a deeper problem than just typical spamming activity. It typically signals that your network, or a device within it, has been compromised by malware or is part of a botnet. Without addressing the underlying compromise, delisting becomes a futile exercise, as the offending traffic will simply re-trigger the blocklist (or blacklist) listing.
The Spamhaus Exploits Blocklist (XBL) is a real-time database of IP addresses that have been observed to be compromised by various exploits, including botnets, worms, and viruses. These infected machines are often used to send spam or launch other malicious activities, typically without the owner's knowledge. Unlike other Spamhaus lists that might focus on deliberate spammers, XBL specifically targets these hijacked systems.
When your IP is listed on the XBL, it means Spamhaus has detected suspicious behavior originating from that IP address. The bare IP address in HELO message is particularly telling. A legitimate mail server should identify itself with a fully qualified domain name (FQDN) in its HELO (or EHLO) command, as specified in RFC 5321 (formerly RFC 2821). A bare IP address often indicates that the sending software is not a properly configured mail transfer agent (MTA) but rather a piece of malware directly connecting to send email.
This kind of repeated listing points to a persistent issue, rather than a one-off spamming incident. It suggests that the source of the malicious traffic remains active on your network. Understanding how email blacklists actually work is crucial here. They are reactive, listing IPs based on observed behavior. If the behavior doesn't stop, the IP will keep reappearing on the blocklist.
The root cause: Compromised systems and misconfigurations
The primary cause of repeated XBL listings for this specific reason is usually compromised machines or misconfigured network devices within your infrastructure. If your mail server is behind Network Address Translation (NAT), this complicates diagnosis. A single infected machine on your internal network can be sending spam through the NAT device, causing your public IP address to be blocklisted.
Consider the scenario where you have multiple devices sharing a single public IP. If just one computer, whether it's a server, workstation, or even an IoT device, gets infected with malware that sends spam, that traffic will egress through your public IP. Since the malware isn't a proper email client or server, it might send out raw SMTP connections with an invalid HELO, triggering the XBL listing. The repeated nature of the listing confirms that the infection or misconfiguration is still present and active.
Another often overlooked aspect is the state of your mail server itself. Even if it's not the primary source of the botnet traffic, an insecurely configured mail server could be acting as an open relay, allowing spammers to send mail through it, or it might have a vulnerability that has been exploited. This would also contribute to your IP appearing on a blocklist. Sometimes, it's not directly your mail server sending the spam, but other devices on your network that are. This can be especially true if you are on shared hosting where someone else's activity impacts your IP.
The message about SMTP connections with HELO values that use a bare IP address is a technical clue. It means that whatever is sending email from your IP is not properly identifying itself. This is typical of malware programs that are not full SMTP clients but rather simple scripts designed to quickly send out large volumes of unsolicited email. They often lack the sophistication to properly form HELO commands with an FQDN, which is a key indicator to blocklist operators like Spamhaus.
Steps to diagnose and remediate
To effectively resolve repeated XBL listings, you must find and eliminate the source of the malicious traffic. Simply requesting delisting without fixing the underlying issue is a temporary band-aid and will lead to immediate relisting. Here's a systematic approach:
Identify the source: This is the hardest part. If you're behind NAT, you need to identify which internal device is generating the traffic. This often involves monitoring your network for unusual outbound SMTP connections (port 25). Look for devices making direct SMTP connections that aren't your designated mail server. Deep packet inspection or firewall logs can be invaluable here. If you're on a single server, run thorough malware scans.
Clean compromised systems: Once identified, the compromised machine needs to be cleaned or, ideally, reimaged. Simply removing malware might not be enough; sometimes, backdoors remain.
Secure your network: Implement firewall rules to prevent any machine other than your official mail server from making outbound connections on port 25 (SMTP). This is critical, especially behind NAT. Also, ensure your mail server isn't an open relay.
Request delisting: After you're confident the problem is resolved, you can use the Spamhaus delisting tool. Be honest about your efforts and the steps taken. Requesting delisting prematurely will only hurt your reputation with them.
For a comprehensive guide on resolving general Spamhaus listings, you might find more details in our article on what causes Spamhaus blocklisting and how to resolve it. Remember, the key is persistent monitoring and immediate action when suspicious outbound SMTP traffic is detected.
Preventing recurring blocklists
Preventing future (or recurring) blocklist appearances requires a proactive approach to your network and email infrastructure security. This is not a set-it-and-forget-it task, especially with the evolving threat landscape.
Regularly scan all network devices for malware and vulnerabilities. Ensure that all operating systems and software, including your mail server software, are kept up-to-date with the latest security patches. Strong password policies and multi-factor authentication for sensitive systems can also prevent unauthorized access that could lead to compromise. For robust protection, consider solutions that provide real-time blocklist monitoring so you're alerted immediately if your IP or domain gets listed.
Crucially, enforce strict outbound firewall rules. If only your designated mail server should be sending email on port 25, then block all other internal machines from doing so. This is a common and highly effective way to prevent compromised internal hosts from causing issues. Remember, even legitimate email senders can end up on blocklists due to underlying network issues.
Views from the trenches
Best practices
Implement strict outbound firewall rules to limit port 25 access to only your dedicated mail server.
Regularly scan all servers and workstations on your network for malware, viruses, and botnet infections.
Ensure your mail server is properly configured with an FQDN in its HELO/EHLO commands and not just an IP address.
Common pitfalls
Repeatedly delisting your IP from Spamhaus XBL without identifying and fixing the root cause of the infection.
Assuming that a persistent blocklist problem is a Spamhaus error rather than an internal network compromise.
Failing to audit all devices behind a NAT, as any infected device can trigger the public IP to be listed.
Expert tips
If your mail server is behind a NAT, strongly consider moving it to a public IP to isolate it from internal network compromises.
Use network flow data (NetFlow, sFlow) to pinpoint the exact internal IP address generating the suspicious SMTP traffic.
For persistent issues, it may be necessary to completely reimage affected machines rather than just cleaning them.
Expert view
Expert from Email Geeks says that repeated Spamhaus XBL listings due to bare IP HELO values are a clear sign of malware or a compromised machine on your network.
2021-07-29 - Email Geeks
Expert view
Expert from Email Geeks says that if your mail server is behind a NAT, compromised machines on the same network are likely spewing spam from your public IP address.
2021-07-29 - Email Geeks
The path forward
Persistent Spamhaus XBL listings indicate a severe and active compromise within your network, rather than a minor deliverability issue. The specific error concerning bare IP HELO values points directly to automated, malicious software (botnets or malware) attempting to send email from your IP address. This is not something that Spamhaus can fix for you, or that repeated delisting requests will resolve without addressing the source.
Your priority must be to thoroughly investigate your network, identify the compromised machines, clean them, and implement robust security measures, particularly strict outbound port 25 filtering at your network's edge. Only once the malicious activity has ceased will your IP address remain off the Spamhaus blocklist (or blacklist) and your email deliverability return to normal. This requires a strong collaboration between your email and network teams.
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