What common blocklists or services cause 'message rejected as spam' errors for ESPs?
Michael Ko
Co-founder & CEO, Suped
Published 13 Aug 2025
Updated 16 Aug 2025
7 min read
For Email Service Providers (ESPs), few messages are as frustrating as 'message rejected as spam'. This generic bounce often hides a complex underlying issue: your emails are being blocked by an anti-spam service or an email blocklist (also known as a blacklist). When this happens, it's not just a single email failing, but potentially a significant portion of your outbound mail being denied, directly impacting your clients' deliverability and your sender reputation.
The challenge lies in identifying which specific blocklist or service is responsible, as the bounce message itself rarely provides enough detail. These rejections can stem from various sources, ranging from well-known public blocklists to proprietary anti-spam filters used by individual Internet Service Providers (ISPs) and mailbox providers.
Blocklists, or blacklists, are essentially databases of IP addresses and domains known to send unsolicited email or exhibit suspicious behavior. ISPs and mailbox providers consult these lists in real time to decide whether to accept an incoming message. Being listed can significantly hamper email deliverability, often resulting in messages being rejected outright or routed to the spam folder. Understanding how email blacklists work is crucial for any ESP.
These lists can be IP-based, blocking messages from specific sending IP addresses, or domain-based, targeting sender domains (like your client's domain or your tracking domain) that have been associated with spam. Some blocklists even compile data on malicious URLs found within email content, flagging emails containing those links regardless of the sending IP or domain. The effect of being listed on a domain-based blocklist can be just as severe as an IP listing.
Beyond traditional blocklists, many ISPs and mailbox providers employ proprietary anti-spam systems that analyze a multitude of factors, including sender reputation, content analysis, and user feedback, to filter incoming mail. These systems often operate on complex algorithms that are not publicly disclosed, making troubleshooting particularly challenging for ESPs.
Key blocklists and services impacting ESPs
When an ESP's messages are rejected as spam, certain blocklists and anti-spam services are frequently implicated. Spamhaus, for example, is one of the most influential. Their lists, such as the Spamhaus Block List (SBL), Exploits Block List (XBL), and Domain Blocklist (DBL), are widely used by mailbox providers globally. If your IP or domain appears on a Spamhaus list, it will significantly impact your deliverability to a broad range of recipients, as their Domain Blocklist specifically targets domains exhibiting spam or malicious activity. We have seen many cases of ESPs getting their customer domains listed in Spamhaus.
Other prominent blocklists include SpamCop and Barracuda Central. SpamCop's blocklist is known for its aggressive approach, often listing IPs based on a single spam complaint. Barracuda Central, on the other hand, is a reputation system used by Barracuda Networks' spam firewalls. If your ESP's IP or domain is listed on these, it can lead to rejections from organizations utilizing their services. Knowing the different types of email blocklists is essential for targeted remediation.
Additionally, many large mailbox providers and enterprises use their own internal anti-spam systems and threat intelligence feeds. For instance, services like Vade (formerly Vade Secure) are increasingly popular. Vade offers a predictive email defense solution used by many ISPs, including those listed in the initial problem statement (like Free.fr, Orange.fr). Their 'Threat List' can block emails based on a combination of IP reputation, domain reputation, and sophisticated content analysis. These proprietary systems are often the culprits behind rejections that aren't immediately apparent on public blocklist checkers.
Common reasons for listing and rejection
The message 'rejected as spam' often indicates a breakdown in sender reputation, which is influenced by numerous factors. High spam complaint rates are a major red flag, as mailbox providers use this feedback to quickly identify and block senders perceived as problematic. Similarly, sending to invalid or old email addresses (which can turn into spam traps) signals poor list hygiene and can lead to blacklisting or blocking.
Beyond list quality, email content itself plays a significant role. Subject lines with excessive capitalization or spammy keywords, unusual formatting, or a high ratio of images to text can trigger spam filters. Furthermore, if your emails contain problematic or blacklisted links (even if they are from a shared click tracker provided by your ESP), this can also lead to rejection messages. This is especially true for shared infrastructure, where one bad actor can impact many legitimate senders.
Finally, incorrect or missing email authentication records, such as SPF, DKIM, and DMARC, can severely undermine your sender reputation. Mailbox providers increasingly rely on these protocols to verify sender legitimacy. Failure to align with these standards often results in emails being flagged or rejected, as it makes your mail appear unverified or spoofed. This is fundamental to a strong email domain reputation.
Troubleshooting and delisting strategies for ESPs
When facing 'message rejected as spam' errors, the first step is always to analyze the bounce messages and email logs. These logs often contain clues, such as the specific error code or the name of the blocking service. Even vague messages like 'policy (a.b.c.d) ID ({hash}) - Your message looks like SPAM or has been reported as SPAM' can hint at content or fingerprint-based filtering if your IPs aren't publicly listed.
Example bounce messagetext
554 5.7.1 Service unavailable; Client host [X.X.X.X] blocked using combined.mail.abusix.zone; https://lookup.abusix.com/search?q=X.X.X.X
Next, use a reliable blocklist checker to verify if your sending IP addresses or domains are listed on any major public blocklists. While MXToolbox is a good starting point, some private or less public blocklists might not be included. If a listing is found, follow the specific delisting procedures outlined by each blocklist operator. Many, like Spamhaus, have clear processes for removal, provided you've addressed the underlying cause of the listing.
For issues with proprietary anti-spam services like Vade, where a public listing isn't visible, proactive communication with the affected ISP can be necessary. Often, this involves filling out a delisting request form on their postmaster site or directly contacting their abuse desk. Most importantly, identify and rectify the root cause, whether it's poor list hygiene, content issues, or authentication failures. Continuous blocklist monitoring is key to staying ahead of these issues.
Views from the trenches
Best practices
Maintain pristine email lists and regularly remove invalid or inactive addresses.
Implement robust email authentication protocols (SPF, DKIM, DMARC) for all sending domains.
Monitor major blocklists and internal ISP reputation signals constantly.
Ensure your email content is relevant, engaging, and avoids spammy characteristics.
Common pitfalls
Ignoring subtle bounce messages that indicate content or fingerprint blocking, not just IP listings.
Assuming an IP is clean if it's not on the most common public blocklists.
Failing to segment lists, leading to high complaint rates from disengaged subscribers.
Not warming up new IPs or domains properly before sending high volumes.
Expert tips
Regularly review bounce logs for specific ISP-generated error codes to pinpoint the exact blocking service.
Engage directly with postmaster teams of affected ISPs for delisting requests, providing clear evidence of remediation.
Utilize internal tools to identify and quarantine senders on shared IPs who are generating excessive spam complaints.
Educate clients on maintaining good list hygiene and content best practices to prevent future issues.
Expert view
Expert from Email Geeks says multiple ISPs, including those listed like Free.fr and Orange.fr, likely utilize Vade as their anti-spam solution, which could explain the rejections.
December 21, 2021 - Email Geeks
Expert view
Expert from Email Geeks says the rejection message implies that the blocking might be content or fingerprint-based, so checking the email's links and the sending IPs thoroughly is crucial.
December 21, 2021 - Email Geeks
Maintaining a strong email reputation
For ESPs, managing email deliverability is a continuous effort, especially with the ever-evolving landscape of anti-spam technologies. When clients encounter 'message rejected as spam' errors, it's a critical signal to investigate deeper than just public IP blocklists. Often, the issue lies with sophisticated anti-spam services or internal ISP filters that analyze email content and sender behavior.
By diligently monitoring bounce logs, maintaining impeccable sender reputation through consistent list hygiene, adhering to authentication standards (SPF, DKIM, DMARC), and proactive engagement with ISPs, ESPs can significantly reduce these rejections. This comprehensive approach ensures higher inbox placement rates and helps maintain trust with both your clients and mailbox providers.
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
Spamhaus, for example, is one of the most influential. Their lists, such as the Spamhaus Block List (SBL), Exploits Block List (XBL), and Domain Blocklist (DBL), are widely used by mailbox providers globally. If your IP or domain appears on a Spamhaus list, it will significantly impact your deliverability to a broad range of recipients, as their Domain Blocklist specifically targets domains exhibiting spam or malicious activity. We have seen many cases of ESPs getting their customer domains listed in Spamhaus.
SpamCop and 
Barracuda Central. SpamCop's blocklist is known for its aggressive approach, often listing IPs based on a single spam complaint. Barracuda Central, on the other hand, is a reputation system used by Barracuda Networks' spam firewalls. If your ESP's IP or domain is listed on these, it can lead to rejections from organizations utilizing their services. Knowing the different types of email blocklists is essential for targeted remediation.
