Why are my emails triggering Gmail phishing warnings and how can I fix it?

Key findings

• Discrepant links: A primary cause of phishing warnings is when the visible text of a link (the hostname) does not match the actual URL it points to. Gmail’s systems are designed to detect such discrepancies, which are a common tactic in malicious phishing attempts.
• Content and HTML issues: While less common than link issues, certain HTML errors, hidden content, or suspicious keywords within the email body can contribute to a phishing flag. Removing essential attributes like alt tags, for instance, might raise suspicions, even if not a direct cause.
• Domain reputation and history: The sender's domain reputation plays a significant role. If a domain has a history of suspicious activity or is associated with poor-quality shared hosting environments, Gmail is more likely to flag its emails. This extends to the reputation of any domains linked within the email.
• User engagement metrics: Gmail heavily relies on user engagement. Even if an email has technical flaws, high positive engagement (opens, replies, forwards, moving from spam to inbox) from your actual subscribers can override initial filtering decisions. Conversely, low engagement can lead to messages being marked as spam or dangerous.
• Authentication standards: Proper implementation of email authentication protocols like SPF, DKIM, and DMARC is foundational to building sender trust and avoiding these warnings.

Key considerations

• Review all links: Thoroughly check all URLs in your email content, especially when using ESP click tracking features. Ensure the visible text aligns with the destination domain. This is a common point of failure that Gmail actively scrutinizes (see Google's recommendations on securing against phishing).
• Monitor domain reputation: Regularly check your domain's reputation using tools like Google Postmaster Tools. This provides insight into how Gmail views your sending behavior and if there are any underlying issues that could contribute to warnings or poor deliverability.
• Content hygiene: Avoid HTML and CSS tricks designed to hide content. Ensure your email content is clear, concise, and does not mimic phishing attempt language or suspicious requests for personal information.
• Prioritize user engagement: Focus on sending relevant, wanted emails to engaged subscribers. Their positive interactions are the strongest signal to Gmail that your emails are legitimate, even outweighing some minor technical imperfections.
• Implement a plain-text version and List-Unsubscribe header: Providing a plain-text alternative and the List-Unsubscribe header improves email hygiene and compliance, which can indirectly contribute to better sender reputation.

Key opinions

• Link structure is paramount: Marketers frequently point to links as the primary culprit. Discrepancies between the displayed URL and the actual destination (often due to click tracking) are highly suspicious to Gmail.
• Template changes can introduce issues: Even seemingly minor design or HTML changes in a new template can inadvertently introduce elements that trigger Gmail's phishing detection algorithms.
• Testing tools have limitations: While useful for initial checks, many deliverability testing tools may not accurately reflect real-world Gmail inbox placement or phishing warnings, especially because they can't replicate individual user engagement.
• Brand and content context: The use of certain brand names or content that mirrors phishing campaigns (even by accident) can raise red flags with Gmail's filters.
• Importance of alt tags: Although not directly tied to phishing warnings, removing alt tags for byte saving is generally discouraged as it can impact accessibility and, indirectly, perceived quality.

Key considerations

• Inspect all external links: If a warning appears, systematically review every link for potential issues, including pointing to poor quality shared hosting or compromised machines, or if you are linking to login pages in a suspicious way.
• Gradual troubleshooting: If the cause isn't immediately obvious, try removing or modifying elements of the email (e.g., specific links, sections of HTML) one by one to isolate the trigger.
• Re-evaluate domain reputation: Check the reputation of your sending domain and any linked domains. A poor reputation can make emails more susceptible to flags. For more detail, read about why emails go to spam.
• Don't over-rely on generic tests: While some tools provide a spam score, they cannot fully replicate Gmail's dynamic filtering, which considers individual user engagement and historical interactions. Focus on your actual audience's experience.

Key opinions

• Deceptive links are a major flag: Using hostnames in the visible text of a link while pointing to a different hostname in the href attribute is considered a very bad practice and is highly likely to trigger phishing warnings.
• Link tracking services: Even when using legitimate ESP click tracking features that redirect links, if the displayed text gives a false impression of the destination, Gmail's machine learning engine will be unhappy. This is a common challenge that needs careful management.
• User engagement is paramount: For Gmail, the most important factor in deliverability is how individual subscribers interact with your emails. High engagement rates can lead to inbox placement even if some technical issues are present, while low engagement can push emails to spam or trigger warnings.
• Limitations of probe accounts: Testing email deliverability using probe accounts (or generic testing services) can be misleading. These accounts do not replicate the historical engagement patterns of your actual subscribers, leading to inaccurate results.
• Hidden content: Coding practices that attempt to hide content using HTML and CSS can be interpreted as deceptive by Gmail and result in emails being marked as spam or dangerous.

Key considerations

• Correct link formatting: Always ensure that the visible text of your links accurately reflects their destination, especially when using third-party click tracking. For more context, see our discussion on preventing phishing warnings.
• Focus on subscriber value: Instead of chasing perfect scores from testing tools, concentrate on sending valuable content that your subscribers genuinely want to receive. Their positive interactions will naturally improve deliverability and mitigate warnings over time. Learn how to boost deliverability rates.
• Patience is key: After implementing fixes, give Gmail's systems a few days to process the changes and observe new engagement patterns. Instant fixes are rare.
• Beware of misleading advice: Some deliverability tools or consultants might offer advice (e.g., shortening links) that, while technically true, does not directly impact deliverability or phishing warnings. Prioritize fixes for core issues identified by mailbox providers (see Gmail Phishing Prevention for more).

Key findings

• Emphasis on email authentication: Documentation frequently stresses the necessity of correctly configuring SPF, DKIM, and DMARC to prove sender legitimacy and prevent impersonation, which is a core component of phishing prevention.
• Content transparency: Official guidelines warn against using HTML and CSS to hide content, as this is a common tactic used in spam and phishing campaigns and can lead to messages being marked as dangerous.
• Link verification: Mailbox providers actively scan links for harmful content. Warnings are often triggered if links point to suspicious domains, contain malware, or are part of known phishing patterns.
• Advanced protection settings: Gmail and similar services have advanced phishing and malware protection settings that scrutinize emails more deeply for risky content and suspicious links before delivery.
• Domain and IP reputation: The reputation of the sending IP and domain is a foundational element in how emails are evaluated, with a poor reputation increasing the likelihood of filtering or warnings.

Key considerations

• Strengthen authentication: Ensure SPF, DKIM, and DMARC records are correctly configured and aligned. This is crucial for verifying your identity and mitigating spoofing attempts. Check for common issues like DMARC verification failures.
• Transparent linking: Avoid any misleading link text. The displayed text of a link should clearly indicate its true destination. This builds trust with recipients and email providers alike.
• Content compliance: Adhere to anti-spam compliance guidelines, avoiding keywords or formatting that could be misinterpreted as malicious. Pre-delivery message scanning is common, so legitimate emails should be free of suspicious elements.
• Sender reputation management: Actively manage your sender reputation. A consistently good reputation signals to email providers that your emails are trustworthy and reduces the likelihood of them being blocked or flagged. Monitor your reputation using platforms like secure Google Workspace solutions.