Why did Gmail mark an internal email as potentially dangerous?
Matthew Whittaker
Co-founder & CTO, Suped
Published 12 Aug 2025
Updated 17 Aug 2025
7 min read
It can be unsettling when an internal email, sent by a colleague within your own organization, suddenly gets flagged by Gmail with a "This message seems dangerous" warning. This alert is typically associated with external threats, so seeing it applied to an email from a trusted internal source raises immediate questions. Why would Gmail treat an email from a coworker, on your own domain, as suspicious?
The common assumption is that internal emails are inherently safe and bypass many of the strict filtering rules applied to external mail. However, Gmail's sophisticated algorithms are designed to protect users from a wide range of threats, including those that might originate or be manipulated within an organization. This means even seemingly innocuous internal messages can sometimes fall under scrutiny. Understanding the factors at play can help clarify why these warnings appear and how to prevent them.
Gmail's advanced filtering and internal emails
Gmail's filtering system doesn't operate on a simple whitelist/blacklist (or blocklist) basis for internal domains. While internal emails generally enjoy a higher trust level, they are not entirely exempt from analysis. The system uses complex machine learning models to identify patterns indicative of phishing, malware, or other malicious content. These models analyze various aspects of an email, not just its origin.
This means that even if an email comes from a verified internal sender, if its content, structure, or other attributes align with known phishing tactics, it can still trigger a warning. This is particularly true for warnings like "This message seems dangerous," which are often a result of these predictive algorithms rather than a direct failure of email authentication. These warnings are often a preventative measure to protect users from potential compromise, even if it turns out to be a false positive.
How machine learning models work
Gmail's security models constantly learn from vast amounts of data, including reported spam and phishing attempts. When an internal email exhibits characteristics that resemble a suspicious message pattern, even if no explicit malicious link or attachment is present, the system may flag it as potentially dangerous. This is often a false positive due to a unique combination of factors rather than a single trigger.
Why legitimate internal emails get flagged
Several factors can contribute to an internal email being flagged. It's rarely one isolated element but often a combination that crosses a threshold for Gmail's detection systems.
One common culprit is the email's content. A subject line that uses overly aggressive sales language, urgent calls to action, or mimics phishing attempts (even if unintentionally) can be a red flag. For instance, words often found in spam or phishing emails, even without malicious links, can raise suspicion. Similarly, unusual formatting, embedded images that load from suspicious external sources, or large, unusual attachments can also contribute. Even a simple company logo in a signature, if embedded in an unusual way, could sometimes be misinterpreted, though this is less common.
Email authentication failures are another critical factor. Even for internal emails, robust SPF, DKIM, and DMARC records are essential. If an email within your domain fails one of these checks, it can indicate a potential spoofing attempt or a misconfiguration. Gmail might see this as a sign that someone is trying to impersonate an internal sender, leading to a warning. This is also why Google and Microsoft have tightened their email authentication requirements.
Content vs. technical causes
Aggressive language: Subject lines or body text containing common spam phrases, urgent demands, or unusual financial terms can trigger flags.
Unusual formatting: Overuse of capitalization, strange characters, or hidden text.
Embedded elements: Images or links that point to suspicious or unindexed domains, even if they're benign.
Attachments: Certain file types or unusually large attachments can be flagged.
Troubleshooting and technical investigation
When an internal email is flagged, the first step is to investigate the specifics of that email. It's easy to assume it's an anomaly, but understanding the root cause is crucial for preventing recurrence and maintaining your domain's email health.
Your IT department should have access to the mail audit logs within Google Workspace. These logs can provide specific reasons why an email was flagged, such as content filters, authentication failures, or even user-reported phishing. This information is invaluable for diagnosing whether it was a false positive or if there's an underlying issue that needs to be addressed.
It's also beneficial to examine the email headers for authentication results. Look for fields like Authentication-Results to see the status of SPF, DKIM, and DMARC. If any of these show a 'fail' or 'softfail' status, it indicates a configuration problem that needs immediate attention. Even if they pass, a poor sender reputation can still cause warnings.
Preventing future warnings
To prevent future internal email warnings, a multi-faceted approach is most effective. It involves both technical configurations and user education.
Firstly, ensure your email authentication protocols are robust and correctly configured. Regularly review your DMARC reports, which provide insights into how your emails are being authenticated and handled by receiving servers like Gmail. If you see authentication failures, address them promptly. This might involve updating SPF records to include all legitimate sending sources, ensuring DKIM signatures are correctly applied, and strengthening your DMARC policy over time. Strong authentication signals to Gmail that your emails are legitimate and not spoofed.
Secondly, educate your team on email content best practices, even for internal communications. While spam trigger words aren't a definitive blocklist, certain phrases or structures can contribute to an email being flagged by Gmail's AI. Encourage clear, concise subject lines and avoid language that might resemble common phishing or marketing spam. If your IT department conducts internal phishing simulations, ensure employees are aware of the purpose and how to interact with genuine internal communications versus simulated threats.
For ongoing monitoring, leverage Google Postmaster Tools. While primarily for bulk senders, it offers valuable domain reputation data that can sometimes highlight underlying issues affecting even internal mail. Pay attention to spam rates and domain reputation metrics. A consistent pattern of internal emails being marked as suspicious could indicate a broader issue with your domain's trust signals, even if your authentication records appear to be in order.
Keeping internal communications secure
An internal email flagged as potentially dangerous by Gmail isn't necessarily a sign of a breach, but it's a clear indicator that something in the email's characteristics triggered a security protocol. By understanding how Gmail's advanced filters work, examining email details, and proactively maintaining your email authentication, you can significantly reduce the likelihood of these warnings. This ensures smooth and secure internal communication within your organization.
Ultimately, a suspicious email warning serves as a reminder that even internal communications are subject to scrutiny in today's threat landscape. Prioritizing email security and deliverability best practices for all mail, regardless of origin, will safeguard your domain's reputation and ensure your messages reach their intended recipients without unnecessary flags.
Views from the trenches
Best practices
Always ensure your DMARC, SPF, and DKIM records are correctly configured and monitored, as authentication failures can lead to internal flags.
Regularly review your Google Workspace mail audit logs to understand the specific reasons why an email was flagged as dangerous.
Educate internal users about email content best practices, avoiding overly aggressive or spam-like subject lines and body text.
Conduct periodic email deliverability checks to assess your overall domain health and identify potential issues before they cause warnings.
If your IT team runs phishing simulations, ensure employees are clear on how to differentiate them from genuine internal communications.
Common pitfalls
Assuming internal emails are completely exempt from Gmail's advanced spam and phishing detection algorithms.
Ignoring
Authentication-Results
in email headers, which can reveal underlying authentication failures even for internal senders.
Dismissing a "This message seems dangerous" warning as a one-off glitch without investigating the root cause.
Expert tips
Machine learning models driving these warnings produce 'maybe' results, making it difficult to pinpoint a single cause like a specific word or link.
The content or structure of an internal email can appear similar to known spam or phishing, even if there's no malicious intent, leading to flags.
Even if the email reached the inbox, the warning indicates a significant concern from Gmail's side, prompting further investigation.
User feedback, such as marking an internal email as 'looks safe,' is used by Gmail to refine its detection models over time.
Sometimes, it's not a technical failure but simply that the email's style and language resemble common spam tactics.
Marketer view
Marketer from Email Geeks says a coworker sent an internal email with a very spammy subject line, which led Gmail to add a 'This message looks dangerous' warning, even though there were no links.
2023-10-04 - Email Geeks
Marketer view
Marketer from Email Geeks says it's not a typical spam filter issue but likely looked concerning enough for Google to highlight it.
