What to do when emails are blocked by major ISPs despite passing DMARC, SPF, DKIM?

Key findings

• Reputation is key: ISPs rely heavily on sender reputation, which is built on consistent positive engagement, low spam complaints, and good list hygiene, rather than just technical authentication passes.
• Beyond authentication: While SPF, DKIM, and DMARC are foundational, they only prove you are who you say you are. They do not guarantee inbox placement if your sending behavior is deemed undesirable.
• User complaints drive blocks: High complaint rates are a major red flag for ISPs, indicating that recipients do not want your mail, regardless of technical compliance.
• Engagement matters: Low open rates and click-through rates, coupled with high unsubscribes, signal to ISPs that your content is not relevant or desired, leading to filtering.

Key considerations

• Diagnose the root cause: Use tools like Google Postmaster Tools and Microsoft SNDS to identify specific issues affecting your domain and IP reputation.
• Review list acquisition: Ensure all email addresses are obtained with explicit consent to prevent high complaint rates. Purchased or outdated lists are detrimental.
• Focus on engagement: Implement strategies to increase subscriber engagement, such as segmenting lists and personalizing content, which helps build a positive sender reputation. See our guide on how to fix emails going to spam.
• Monitor and react: Continuously monitor your deliverability metrics and bounce messages. Swiftly address any issues identified to prevent further blocks or blacklisting.

Key opinions

• Tools are a starting point: Marketers often begin with free tools like Google Postmaster Tools and Microsoft SNDS/JMRP to gain initial insights into deliverability issues.
• Header analysis: Checking email headers for spam weight tags or specific bounce codes is a common first step to diagnose the problem at a granular level.
• Personalization impact: Some marketers have successfully resolved Gmail blocking issues by increasing email personalization, suggesting content relevance and uniqueness play a role.
• ESP responsibility: If using an Email Service Provider (ESP), managing feedback loops is often their responsibility, simplifying some technical burdens for marketers.

Key considerations

• Investigate bounce reasons: Pay close attention to the specific bounce messages received from ISPs, as these often contain critical clues about why emails are being blocked. Our guide on troubleshooting transactional emails going to spam provides further detail.
• Beyond technical checks: Even if Litmus or similar tools show no blacklisting and authentication passes, broader reputation factors are at play. Consider a more comprehensive email deliverability test.
• Gmail-specific resolution: For Gmail blocks, utilize the bulk sender contact form, which has been effective for warming up IPs and resolving false positives.
• Infrastructure review: Understanding your sending infrastructure and providing full email headers to deliverability experts can help pinpoint issues.

Key opinions

• Reputation over authentication: The primary reason for blocks, even with correct authentication, is usually poor sender reputation, driven by recipient behavior like complaints.
• Consent is critical: Address collection without explicit recipient permission is a major cause of deliverability issues, leading to spam complaints and blocks.
• Bounce analysis: Analyzing bounce responses is vital for understanding the specific type of block or deferral (e.g., 421 bounces) and diagnosing the issue.
• Beyond legality: Even if an email is legally compliant, if recipients perceive it as unwanted, it will lead to complaints and subsequent blocks.

Key considerations

• Fix list acquisition practices: Prioritize reviewing and correcting any methods that collect email addresses without clear consent. This often involves ensuring double opt-in processes.
• Clean unengaged subscribers: Proactively remove recipients who have not engaged with your emails for a long period, as they are likely to generate complaints or land in spam traps. Read our guide on spam traps.
• Understand ISP algorithms: Recognize that ISPs use complex algorithms that factor in engagement and complaint rates alongside authentication. Our blog post on email deliverability issues provides further insight.
• Maintain positive sender behavior: Continuously strive for positive user interaction and minimal spam complaints to build and protect your sender reputation.

Key findings

• DMARC policy enforcement: Documentation confirms that DMARC policies dictate how recipient mail servers handle emails that fail SPF and DKIM authentication, ranging from monitoring to outright rejection. Our guide on DMARC record and policy examples provides further details.
• Authentication basics: SPF, DKIM, and DMARC are consistently described as foundational protocols for email authentication, designed to verify sender identity and prevent malicious activity like spoofing. For more information, see our guide on DMARC, SPF, and DKIM.
• DKIM signing importance: Documentation often highlights the necessity of implementing DKIM signing for all outgoing emails and regularly monitoring DNS records to prevent unauthorized changes that could impact deliverability.
• DMARC pass conditions: Official sources clarify that DMARC will pass if either SPF or DKIM aligns and passes, failing only if both authentication methods do not align or fail.

Key considerations

• Beyond authentication: While authentication is fundamental, ISPs also consider sender reputation built on engagement and complaints. Implement robust email security frameworks that encompass more than just DMARC configuration.
• Holistic deliverability view: Documentation emphasizes that DMARC should be a key component of a broader email security and deliverability strategy, not a standalone solution.
• Monitor and troubleshoot: Regularly review DMARC reports and troubleshoot any authentication or alignment issues that arise. Our guide on DMARC verification failed errors can help.