Why is Google marking its own emails as dangerous?

Key findings

• URL inconsistencies: The most common theory suggests that warnings arise from suspicious or unexpected URLs embedded within the message, even if the email sender appears legitimate.
• Content flags: Emails can be flagged if their content, regardless of the sender, contains elements that Google's spam filters (which are often AI-driven) identify as potentially harmful or deceptive. This applies even if the email appears to be from a trusted source.
• Forged invitations and abuse channels: A common vector is abuse through services like Google Calendar invitations, where malicious actors exploit legitimate features. Google's systems can detect when an invite is not genuinely generated by the system, or when it's being used for spam.
• Recipient protection: Google's filters prioritize protecting the recipient. Therefore, if there's any doubt about a domain's reputation or message content, Gmail will likely issue a warning to safeguard its users, even if the email technically passes authentication checks like DMARC.

Key considerations

• Examine headers: To truly understand why an email was flagged, a deep dive into the full email headers is necessary. This provides crucial authentication details and routing information that can reveal anomalies. You can also monitor your domain's health using Google Postmaster Tools.
• Authentication standards: Ensure that your email authentication, including SPF, DKIM, and DMARC, is correctly configured. While these may pass, other signals can still trigger warnings. Gmail's filters are increasingly sophisticated, using a variety of signals beyond just authentication to determine what gets marked as spam. More information on Gmail's spam filters can be found on the Google Workspace Blog.
• Evolving threats: The methods used by malicious actors constantly evolve, leading Google to update its security measures frequently. What might be deemed safe today could be flagged tomorrow due to new detection patterns.

Key opinions

• URL suspicion: Many marketers quickly point to the URLs contained within the message as the primary suspect. They believe Google's filters are hyper-sensitive to unusual or suspicious links, even if the email comes from a known sender.
• Not truly Google content: A common belief is that if an email from Google is flagged, it's likely not genuinely generated by Google's core systems but rather an exploited feature, such as calendar invitations being used for spam or phishing.
• Domain or content issues: Marketers recognize that problems with the sending domain's reputation or the email's content (e.g., specific keywords, formatting) can cause Gmail to flag it as dangerous. Google prioritizes protecting the recipient from potential harm.
• False positives: There's a general understanding that Gmail's warnings, while helpful, can sometimes be false positives, especially when a system (like mail forwarding for a newly registered domain) triggers an unexpected behavioral pattern.

Key considerations

• Link scrutiny: Marketers must carefully review all links in their emails for suspicious redirects, shortened URLs, or anything that could be misinterpreted by a spam filter. You can learn more about why emails trigger phishing warnings.
• Authenticity matters: Even if an email appears to come from a reputable source, its internal structure and adherence to email standards are crucial. Deviations can lead to flags, underscoring the importance of robust sender authentication.
• Content best practices: Understanding how Google flags CDN or email content as malicious is vital. Avoid spammy keywords, unusual formatting, or excessive images that can trigger filters. Resources like WP Mail SMTP offer advice on fixing 'Be Careful With This Message' errors.

Key opinions

• DMARC pass isn't enough: Experts highlight that while a DMARC 'pass' is crucial, it doesn't automatically mean an email is safe. Google's systems look beyond authentication headers at content and behavioral patterns to determine trustworthiness.
• Invite abuse and replay attacks: The flagging could be due to potential invite abuse or a 'replay attack,' where legitimate features are misused or emails are re-sent by unauthorized parties.
• Forging with DMARC pass: There are known vulnerabilities where forgeries can still achieve a DMARC 'pass,' exploiting weaknesses in how some mailbox providers handle authentication. This is a complex area of email security.
• Mailbox provider vulnerabilities: Some mailbox providers have vulnerabilities that make it easier for malicious actors to authenticate forged emails, even from seemingly reputable domains, leading to such warnings.

Key considerations

• Advanced threat vectors: Understanding sophisticated phishing and spoofing techniques, including those that bypass standard authentication, is crucial for both senders and recipients. This includes understanding the nuances of DMARC verification failures.
• Full header analysis: Experts consistently recommend analyzing the full email headers (not just the 'from' address) to get a complete picture of an email's origin and authentication journey. This is key to diagnosing complex deliverability issues.
• Ongoing vigilance: The email security landscape is constantly evolving. Mailbox providers, including Google, continuously update their algorithms to detect new threats, meaning senders must stay informed and adapt their practices. Consistent blocklist monitoring is a good practice to proactively manage your sending reputation and avoid being listed on a blocklist or blacklist.
• Recipient perspective: Mailbox providers err on the side of caution. If there's any perceived risk, they will flag the email to protect their users, even if the sender is technically legitimate but has an unusual sending pattern or content.

Key findings

• AI-driven filters: Gmail employs numerous AI-driven filters that analyze a wide array of signals to determine what constitutes spam or dangerous content. These filters are continuously updated to counteract new threats.
• Multifactor analysis: Filters consider various signals, including sender reputation, content, links, user engagement, and adherence to email standards, not just basic authentication like DMARC. This comprehensive approach helps catch sophisticated attacks.
• Proactive protection: Google's goal is to keep malicious emails out of the primary inbox by automatically marking them as spam or blocking them entirely. Warnings serve as an immediate alert to users about potentially harmful messages.
• Vulnerabilities and exploits: Research highlights that even with strong email authentication protocols, vulnerabilities in Mail User Agents (MUAs) or specific handling by Internet Service Providers (ISPs) can sometimes allow forged emails to be authenticated.

Key considerations

• Holistic deliverability: For optimal deliverability, senders must go beyond basic compliance and focus on maintaining a strong sender reputation, ensuring content relevance, and providing positive user experiences. This includes understanding the impact of your domain reputation on Gmail.
• Dynamic threat landscape: Google's security models are constantly evolving to combat new phishing and spam tactics, particularly during peak times like holidays. Senders should expect continuous adjustments in filtering mechanisms. For more technical insights into email security, consider reviewing research from sources like USENIX.
• User intent and engagement: Beyond technical compliance, user signals (such as whether recipients open, reply, or mark as spam) heavily influence Gmail's filtering decisions. Emails that users don't want to see are consistently demoted or blocked.
• Safe browsing integration: Gmail often integrates with Google Safe Browsing, meaning any suspicious links or domains within an email (even if the email itself passes authentication) can trigger warnings. This is critical for understanding how Google Safe Browsing impacts deliverability.