Why are Gmail emails flagged with 'Images are hidden, this message might be suspicious' banner?

Key findings

• Sender reputation: Even with a seemingly high Sender Score, other aspects of your sender reputation, such as domain history or specific IP performance within a pool, can trigger this warning. Gmail's algorithms are complex and factor in many signals.
• Image and link hosting: The reputation of domains hosting your images and tracking links is crucial. Generic or shared cloud storage URLs (like certain S3 patterns) can sometimes be flagged if they're associated with questionable activity across other senders. Consider how Gmail may be blocking your emails more broadly.
• Gmail's evolving filters: Google continuously updates its spam and security filters. A new banner or a change in flagging behavior can indicate adjustments to how they assess messages, especially concerning suspicious content or unusual sending patterns. This ties into why Gmail shows dangerous alerts.
• Specific email types: Welcome emails or transactional messages might be subject to different scrutiny, especially if they exhibit patterns often seen in spam, even if the sender's overall reputation is good. Refer to our article on why Gmail flags messages due to low sender reputation.

Key considerations

• Content analysis: Review the specific content of the flagged emails for any elements that might appear suspicious. This includes both visible links and hidden tracking pixels.
• Domain and IP reputation: Investigate the reputation of all domains and IPs involved in your email, including those used for image hosting and click tracking. Ensure consistent, strong authentication.
• Branded URLs for images and tracking: Wherever possible, use branded subdomains for image hosting and link tracking to align these with your primary sending domain, enhancing trust signals.
• Authentication standards: Ensure your SPF, DKIM, and DMARC records are correctly configured and aligned, as these are fundamental to establishing sender authenticity.
• Engagement monitoring: Monitor user engagement closely. Low open rates, high delete rates, or spam complaints for specific campaigns can reinforce negative reputation signals with Gmail.

Key opinions

• Recent occurrence: The banner has been noticed more frequently in recent weeks, suggesting a potential shift in Gmail's filtering or display rules for suspicious content.
• CDN and image URL reputation: Concerns were raised about the reputation of image URLs and CDNs (Content Delivery Networks). Hosting images on platforms like AWS S3 could lead to flagging if not configured with branded URLs, as discussed in our piece on why Google flags CDN content as malicious.
• Broken links: Broken image hosting links or inaccessible images are a common cause for such warnings, indicating a technical issue rather than purely a reputation one.
• Tracking link scrutiny: Some speculate that Google might be increasing scrutiny on tracking links, especially in one-to-one or cold outreach emails, even if they are HTTPS. This aligns with warnings about suspicious link notifications for HTTPS websites.
• Specific email types flagged: Welcome emails, even from senders with good overall reputation, sometimes trigger this banner. This could be due to specific patterns associated with onboarding sequences or high initial spam reports for these email types. This reflects how Gmail messages are seen as suspicious.

Key considerations

• Check image accessibility: Ensure all images are publicly accessible and their hosting links are valid. Broken or inaccessible image links can directly contribute to this warning.
• Review tracking setup: Examine your email tracking setup, especially if it involves third-party services. The reputation of their domains or the way tracking pixels are implemented could be a factor.
• Branded CDN URLs: If using a CDN like AWS S3, configure branded URLs (e.g., images.yourdomain.com) instead of generic ones (s3.amazonaws.com) to build a consistent domain reputation across all email elements.
• Isolate the issue: If only specific email types (like welcome emails) are affected, analyze their unique content and sending patterns compared to other campaigns.

Key opinions

• Google's dynamic filtering: Google is constantly adjusting its filtering rules and the messages displayed to end-users. This banner might be a result of them twiddling some dials after rolling out new requirements, potentially as a way to train their spam filters.
• IP reputation impact: Even within a pool of IPs, one IP might be flagged while others sending identical content are not, suggesting highly granular (and sometimes inconsistent) IP-level reputation issues. Learn more about what happens when your IP gets blocklisted.
• Content and image reputation: The reputation of image URLs, especially those from generic or shared domains like s3.*, can affect deliverability. Using branded URLs for image hosting can often resolve these issues.
• Behavioral flagging: Gmail may apply this warning when it detects patterns common in unwanted mail, such as certain cold outreach tactics, even if authentication is sound. This is part of how Gmail determines why your emails are going to spam.
• Bot abuse impact: If a sender has recently been a victim of bot abuse, their mail's fingerprint might be associated with bad behavior, leading to increased scrutiny and warnings like this.

Key considerations

• Monitor specific IPs: If you use multiple IPs, closely monitor the deliverability of each, as issues might be localized to one or a few within your pool.
• Align all domains: Ensure that tracking and hosting domains are aligned with your From domain. This consistency helps build a unified positive reputation, as detailed in our guide to understanding your email domain reputation.
• Review email content fingerprint: Analyze whether the fingerprint of your emails, including hidden elements and overall structure, might be inadvertently mimicking patterns associated with bad behavior or bot abuse.
• Secure tracking images: Verify that all tracking images and pixels are served over HTTPS, even if they are small or seemingly insignificant, as non-secure elements can trigger warnings. This is often related to inconsistent suspicious link warnings.

Key findings

• Sender reputation is paramount: Low sender reputation is a frequently cited reason for emails being flagged as suspicious or spam, even leading to images being hidden. Reputation is built on consistent positive sending behavior and user engagement.
• Authentication is critical: Missing essential DNS records or misaligned DKIM signatures can cause Gmail to flag messages as suspicious. Proper SPF, DKIM, and DMARC configuration is fundamental for proving sender legitimacy.
• Link reputation: Gmail flags links it deems suspicious, which can significantly increase the likelihood of an email landing in the spam folder. This extends to image links and tracking pixels.
• Content analysis for malicious elements: Gmail's filtering aims to protect users from phishing and malware. This includes flagging emails with deceptive content or malicious attachments, which could be disguised within images or embedded links.
• Gmail's spam policies: Google's automatic flagging of emails that meet specific criteria is a core method of user protection. This includes assessment of sender reputation, content, and linked domains. Our guide to Google Postmaster Tools provides key insights.

Key considerations

• Monitor deliverability metrics: Pay close attention to your sender reputation metrics in tools like Google Postmaster Tools. A decline in reputation can directly lead to these warnings. For a comprehensive overview, see our email deliverability test checklist.
• Ensure proper DNS records: Regularly check your SPF, DKIM, and DMARC records for correct configuration and alignment. Misconfigurations can severely impact trust signals.
• Review all linked content: Scrutinize all links and image sources within your emails, ensuring they are from reputable domains and are not flagged as malicious or suspicious by security services. This includes third-party tracking links.
• Avoid suspicious patterns: Design emails to avoid characteristics commonly associated with spam or phishing, even if unintended. This includes deceptive language, overly generic links, or inconsistent branding.