Summary
Gmail's anti-phishing warnings are triggered by a multifaceted approach designed to protect users from spoofing and phishing attempts. Key factors include: sending emails from outside the recipient's organization with a similar sender name (particularly if authentication is weak), emails impersonating internal employees, missing or misconfigured authentication records (SPF, DKIM, DMARC), a poor domain or IP reputation (due to spam activity, blacklisting, high bounce rates, or spam complaints), the use of new domains without a sending history, sudden spikes in email volume, suspicious links or language within the email content, and a domain name's similarity to known phishing domains. Gmail uses various signals and anti-spoofing measures to detect anomalies, flagging messages that might not be from whom they claim to be. Legit senders with proper configurations typically don't need to worry.
Key findings
- External Sender & Name Similarity: Sending from outside the recipient's organization with a sender name similar to someone internal is a major trigger.
- Impersonation Attempts: Emails impersonating employees are flagged as potential phishing attempts.
- Authentication Failures: Missing or incorrectly configured SPF, DKIM, and DMARC records significantly increase the likelihood of warnings.
- Poor Domain/IP Reputation: A negative sending reputation due to spam activity, being blacklisted, or high bounce rates is a critical factor.
- New Domain Sending: New domains without established sending history are more likely to trigger warnings.
- Volume Spikes: Sudden surges in email volume can trigger security alerts and anti-phishing mechanisms.
- Suspicious Content: The use of suspicious links or language within the email content can lead to warnings.
- Domain Name Resemblance: Domains with names similar to known phishing domains are more likely to be flagged.
- Anomaly Detection: Gmail employs various anti-spoofing measures and detects anomalies that suggest the email isn't legitimate.
Key considerations
- Proper Authentication Setup: Implement SPF, DKIM, and DMARC correctly to authenticate your emails and prevent spoofing.
- Maintain Positive Reputation: Monitor and maintain a good domain and IP reputation by adhering to best practices and avoiding spam-like behavior.
- Gradual Domain Warmup: Gradually warm up new domains to establish a positive sending history with Gmail.
- Consistent Sending Volume: Avoid sudden spikes in email volume, and gradually increase sending volume over time.
- Content Review and Optimization: Carefully review email content and avoid using suspicious links, language, or formatting that could be flagged.
- Monitor Blacklists: Regularly check if your domain or IP address is on any public blacklists.
- Monitor Postmaster Tools: Use Google Postmaster Tools to monitor your domain's reputation and identify any potential issues.
- External Sender Awareness: Be aware that sending from outside a recipient's organization, particularly with a similar name, can trigger warnings, and take steps to mitigate this.
What email marketers say
11 marketer opinions
Gmail's anti-phishing warnings are triggered by a combination of factors including sending emails from outside the recipient's organization with a similar sender name, failing authentication checks (SPF, DKIM, DMARC), having a poor domain or IP reputation, being on a public blacklist, using a new domain without a sending history, sending sudden spikes in email volume, or including suspicious content.
Key opinions
- External Senders: Sending from outside the recipient's organization, especially with a similar sender name, triggers warnings.
- Authentication Failures: Missing or misconfigured SPF, DKIM, and DMARC records lead to warnings.
- Reputation Issues: Poor domain or IP reputation due to spam activity or being blacklisted increases the likelihood of warnings.
- New Domains: New domains without a positive sending history are more likely to trigger warnings.
- Volume Spikes: Sudden increases in email volume can trigger security alerts.
- Content: Suspicious links or language can cause warnings.
Key considerations
- Authentication Setup: Ensure proper SPF, DKIM, and DMARC configuration to authenticate emails.
- Domain Reputation: Monitor and maintain a good domain and IP reputation by avoiding spam practices.
- Domain Warmup: Warm up new domains gradually to establish a positive sending history.
- Sending Volume: Avoid sudden spikes in email volume to prevent triggering security alerts.
- Content Review: Review email content for suspicious links or language that might be flagged as phishing.
- Sender Name Similarity: Be aware that similar sender names to internal users can trigger warnings, especially for external senders.
Marketer view
Email marketer from GMass explains that newer domains without a sending history are more likely to trigger anti-phishing warnings in Gmail.
28 Jul 2021 - GMass
Marketer view
Email marketer from Email Vendor Blog shares that warnings might appear if the sending IP address or domain has a poor reputation due to previous spam activity.
19 Jul 2024 - Email Vendor Blog
What the experts say
3 expert opinions
Gmail's anti-phishing warnings are triggered by emails impersonating employees with different email addresses, anomalies suggesting the sender isn't who they claim to be, and a combination of missing authentication (SPF, DKIM, DMARC), poor sender reputation, and phishing-like content. Legit senders are generally unaffected.
Key opinions
- Impersonation: Emails impersonating employees trigger warnings.
- Authentication: Missing or misconfigured SPF, DKIM, and DMARC records contribute to warnings.
- Reputation: Poor sender reputation is a factor.
- Content: Content resembling phishing tactics triggers warnings.
- Anomalies: Detection of anomalies suggesting email is not legitimate.
Key considerations
- Legitimate Senders: Legitimate senders typically don't need to worry, assuming proper configuration.
- Authentication Setup: Ensure SPF, DKIM, and DMARC are correctly configured.
- Sender Reputation: Monitor and maintain a good sender reputation.
- Content Review: Avoid using content that could be mistaken for phishing tactics.
Expert view
Expert from Spam Resource explains that Gmail's phishing warnings are often triggered by a combination of factors, including missing or misconfigured authentication records (SPF, DKIM, DMARC), domain reputation issues, and content that mimics known phishing tactics.
13 Sep 2022 - Spam Resource
Expert view
Expert from Email Geeks explains it's an anti-phishing warning where mail pretends to be an employee by impersonating them but using a different email address. Legit senders shouldn't care or worry about it.
30 Mar 2025 - Email Geeks
What the documentation says
5 technical articles
Gmail uses multiple signals to identify spoofed messages, especially those impersonating internal users, to prevent phishing. Implementing DMARC helps prevent spoofing by providing instructions to mail servers on how to handle failed authentication checks (SPF/DKIM). Maintaining a good domain reputation is crucial. Similarity to known phishing domains increases warning likelihood.
Key findings
- Spoofing Detection: Gmail uses multiple signals to detect spoofed messages.
- Internal Impersonation: Impersonation of internal users is a key trigger.
- DMARC Implementation: DMARC provides instructions for handling authentication failures.
- Domain Reputation: Good domain reputation is crucial for avoiding warnings.
- Domain Similarity: Similarity to known phishing domains triggers warnings.
Key considerations
- Implement DMARC: Implement DMARC to instruct mail servers on handling failed authentication.
- Maintain Reputation: Maintain a good domain reputation through responsible sending practices.
- Avoid Impersonation: Ensure emails don't inadvertently impersonate internal users.
- Domain Name Choice: Choose a domain name that doesn't closely resemble known phishing domains.
Technical article
Documentation from RFC Editor details the technical specifications of DMARC, explaining how it allows domain owners to indicate how email receivers should handle messages that fail SPF or DKIM authentication.
14 Dec 2023 - RFC Editor
Technical article
Documentation from Google explains that Gmail uses multiple signals to identify and flag potentially spoofed messages, especially those impersonating internal users, to prevent phishing.
20 Aug 2022 - Google
How can a phishing email pass SPF and DKIM authentication checks?
How can email senders and users prevent and identify phishing emails?
How can I avoid Gmail security warnings on emails?
How do I troubleshoot Gmail phishing email warnings?
Why are emails being marked as junk or phishing in Outlook 365?
Why is Gmail showing 'This message seems dangerous' warning?
