What common blocklists or services cause 'message rejected as spam' errors for ESPs?

Key findings

• Vade retro: Vade Retro (now Vade) is a frequently cited anti-spam solution used by many ISPs, particularly in Europe (e.g., Free.fr, Orange.fr) and North America (e.g., Cogeco, Windstream). It's known for its content and fingerprint-based filtering, not just IP or domain listings on public blocklists.
• Beyond public blocklists: The absence of an IP or domain on public DNSBLs doesn't mean your emails aren't being flagged. Many ISPs use their own internal reputation systems, spam traps, and proprietary algorithms, which aren't publicly searchable. These internal systems can blacklist IPs or domains based on real-time spam complaints or suspicious patterns.
• Content and fingerprinting: Rejection messages that mention “Your message looks like SPAM” often indicate content-level filtering. This means the message body, subject line, embedded links, or even the overall email structure matches known spam fingerprints, regardless of sender reputation (at least, initially). For more insights, refer to our guide on why emails go to spam.
• Shared infrastructure impact: For ESPs, the issue could stem from shared IP addresses or even shared tracking domains. If another user on the same shared infrastructure sends spam, it can negatively impact all senders using those resources. Learn more about how to address issues related to shared email infrastructure.
• Specific ISPs: ISPs like Free.fr, Cogeco.ca, BTinternet.com (including btopenworld.com, talk21.com), Windstream.net, Orange.fr, and Bigpond.com (Telstra) are known for stringent spam filtering, often employing commercial anti-spam solutions like Vade or their own highly tuned systems. Resolving these requires direct engagement or specific unlisting processes for those services.

Key considerations

• Thorough investigation: When an email is rejected as spam, it's crucial to examine not just the IP reputation but also the content of the message itself, including all links and images. Even minor changes in content can sometimes resolve these rejections.
• ISPs' internal systems: Be aware that many larger ISPs maintain their own private blocklists or internal reputation scoring systems that are not publicly queryable. Getting listed on one of these can be particularly challenging to troubleshoot without direct communication with the ISP's postmaster or the anti-spam vendor they use.
• Vade removal process: If Vade (formerly Vade Retro) is suspected, it is often possible to submit a false positive report via their sender tool. More information on this can be found on SpamResource.
• Engagement with postmasters: For persistent issues, especially with major ISPs, direct communication with their postmaster team is often the most effective route. Provide detailed logs and bounce messages to help them identify the precise reason for rejection.
• Deliverability best practices: Adhering to fundamental deliverability best practices is paramount. This includes maintaining clean mailing lists, sending relevant content, monitoring engagement, and ensuring proper email authentication (SPF, DKIM, DMARC). Regular email deliverability tests can help identify issues proactively.

Key opinions

• Content-based filtering is common: Many marketers suspect that rejections mentioning “looks like SPAM” are more about the email's content, subject line, or linked URLs rather than just the sending IP's reputation.
• Hidden blocklists are a factor: It's commonly believed that ISPs use internal or less visible blocklists and anti-spam services that aren't reported on public tools, making troubleshooting more difficult.
• Shared tracking domains can be problematic: For ESPs, a common pain point is when shared click or open tracking domains used across multiple clients become blacklisted due to one client's poor practices. This is similar to why click tracking links can be blocked.
• Vade is a known challenge: Several marketers specifically mention Vade as a common spam filter that can cause rejections across various ISPs.
• Focus on reputation: Many believe that maintaining a strong sender reputation is key to avoiding these elusive rejections, even if the specific blacklist isn't identified. This includes practices that influence your email domain reputation.

Key considerations

• Analyze bounce messages carefully: The specific wording of the bounce message, especially policy IDs or hash values, can sometimes provide clues to the filtering system being used by the recipient ISP. This is a common point of discussion, such as with Spiceworks users often note.
• Test content variations: If content-based filtering is suspected, marketers should try sending simplified versions of their emails (plain text, no links, minimal formatting) to the problematic domains to isolate the issue.
• Engage ISP postmasters: Even without a known public blocklist, contacting the recipient ISP's postmaster (if they have a public contact or specific form) is recommended. They can often provide more specific reasons for the rejection.
• Monitor hidden signals: Besides public blocklists, marketers should monitor other reputation signals like spam complaint rates, unknown user rates, and engage with tools that provide insight into private blocklists or ISP-specific issues if available.

Key opinions

• Vade is a key player: Experts frequently identify Vade (formerly Vade Retro) as a significant anti-spam service utilized by a wide range of global ISPs, making it a common reason for “message rejected as spam” errors, even without public blocklisting.
• Rejection messages provide clues: The specific phrasing in bounce messages, such as mentioning “policy ID” or “looks like SPAM”, suggests that filtering is often based on content or email fingerprints rather than just IP or domain reputation.
• Shared infrastructure risks: Using shared IP addresses or domains for tracking within an ESP environment increases the risk of being affected by other senders' poor practices, leading to broad rejections. This is covered in our guide on what happens when your IP gets blocklisted.
• Comprehensive approach needed: Troubleshooting rejections requires a multifaceted approach, including examining email content, sender authentication (SPF, DKIM, DMARC), and engagement metrics, not just checking public blocklists.
• ISPs' private systems: It's commonly accepted that major ISPs rely heavily on internal reputation systems and threat intelligence feeds that are not publicly accessible, making direct communication with postmasters essential for resolution.

Key considerations

• Investigate Vade listings: If the affected domains (e.g., free.fr, orange.fr) are known to use Vade, direct investigation via the Vade sender tool (if accessible) is crucial for identifying and remediating any false positive listings. This process is detailed by SpamResource.
• Beyond MXToolbox: Relying solely on common blocklist checkers is insufficient. Experts advise looking at ISP-specific tools, bounce logs, and engaging directly with postmasters for more detailed information when generic tools yield no results.
• Content and link auditing: Regularly audit email content, including all embedded links and images, for anything that might trigger spam filters. This includes checking for suspicious domains, poor formatting, or excessive use of spammy phrases.
• Proactive reputation management: Maintain consistent sending volume, segment lists effectively, and monitor engagement to build and preserve a strong sender reputation. This can help prevent landing on internal blacklists or triggering content filters. Read more in our guide, What does it mean when your email is blacklisted?
• Understand ISP specifics: Each ISP has unique filtering mechanisms. While some use common commercial services, others develop their own. Familiarity with the major ISPs and their known filtering tendencies can aid in quicker diagnosis.

Key findings

• Bounce messages decode: Documentation often explains that specific bounce messages, like “550 5.4.1 Recipient address rejected: Access denied”, are Non-Delivery Reports (NDRs) indicating that the recipient server blocked the message, often due to spam policies.
• Internal reputation services: Many ISPs use internal reputation services that monitor sender behavior and content patterns. These systems can blacklist (or blocklist) senders even if they aren't on public blocklists, as noted by Abusix, which can lead to rejections.
• Spam filters and content analysis: Documentation confirms that spam filters analyze email content, subject lines, attachments, and URLs for characteristics commonly associated with spam. This can lead to rejections even if the sender's IP is not explicitly blocklisted.
• Sender authentication importance: Proper implementation of SPF, DKIM, and DMARC is consistently highlighted as critical. Failures in authentication can significantly degrade sender reputation and result in messages being marked as spam or rejected outright, as emphasized in the Rackspace documentation.
• Recipient-side filtering: User-level blocklists and webmail filter settings on the recipient's end can also cause rejections, even if the sender is otherwise legitimate. This means the problem might be localized to specific recipients rather than a broad ISP block, a point often covered in support guides like OpenSRS.

Key considerations

• Monitor delivery logs: Regularly review detailed delivery logs provided by your ESP or mail server for precise bounce codes and messages. These logs are the first line of defense in identifying the specific reason for rejection.
• Check sender reputation: Beyond public blocklists, monitor your sender reputation through tools like Google Postmaster Tools or similar services offered by other ISPs. These often provide insights into spam rates and domain reputation as seen by the ISPs themselves.
• Content compliance: Ensure email content adheres to anti-spam guidelines. Avoid deceptive subject lines, excessive capitalization, and suspicious links. Regularly audit your email templates and sending practices for compliance.
• Authentication standards: Verify that your SPF, DKIM, and DMARC records are correctly set up and aligned. Authentication failures are a significant red flag for spam filters. Further guidance can be found in our simple guide to DMARC, SPF, and DKIM.
• List hygiene: Maintain a clean and engaged subscriber list to minimize bounces and spam complaints. Remove inactive users and implement confirmed opt-in to ensure recipients truly want your emails, preventing issues like spam trap hits.