Summary
Google's sophisticated filtering systems frequently flag CDN or email content as malicious due to a combination of factors related to sender reputation, the integrity of linked content, and suspicious email practices. Key reasons include a poor domain, IP, or sender reputation often influenced by high spam rates or user complaints. Furthermore, the direct detection of malware, phishing scams, or exploit kits within linked content, even when served by a CDN, is a significant trigger. Suspicious link characteristics, such as long or numeric tracking URLs, domain mismatches where the link text differs from the actual URL, and multiple redirects, can also cause flags. Lastly, a compromised origin server or the absence of proper email authentication, like SPF, DKIM, and DMARC, can lead Google to deem content untrustworthy.
Key findings
- Poor Reputation Triggers Flags: A primary reason for content being flagged is a poor domain, IP, or sender reputation, often stemming from high spam rates, user complaints, or blacklisting. This includes the reputation of the domains hosting CDN assets.
- Malicious Content Detection: Google's systems actively detect actual malicious content, such as phishing scams, malware, unwanted software, exploit kits, or suspicious JavaScript, embedded within linked resources or the email content itself, even if served via a legitimate CDN.
- Suspicious Link Characteristics: Links that are long, numeric, contain domain mismatches (where link text differs from the actual URL), involve multiple redirects, or point to domains with a history of abuse can trigger malicious warnings.
- Compromised Origin Servers/CDNs: If the origin server from which a CDN pulls content is compromised with malware, or if previous content served from a CDN domain was identified as dangerous, all linked content can be flagged due to lingering negative reputation.
- Lack of Email Authentication: Insufficient or improperly configured email authentication (SPF, DKIM, DMARC) can make legitimate emails and their linked content appear suspicious, leading to them being flagged as potentially malicious by strong spam filters.
- Phishing & Deceptive Practices: Email content, including CDN-hosted elements, is flagged if it contains suspicious phrasing common in phishing attempts, is designed to deceive users, or leads to known fraudulent sites.
Key considerations
- Maintain Sender Reputation: Continuously monitor and improve your domain and IP reputation through tools like Google Postmaster Tools, focusing on low spam rates and user complaints.
- Secure Content Hosting: Regularly scan your linked websites, origin servers, and CDN-hosted assets for malware, phishing, suspicious JavaScript, or any signs of compromise. Ensure images are hosted securely, ideally by your sending domain or a trusted ESP that passes authentication.
- Optimize Link Practices: Use clear, concise tracking links that are ideally shorter and non-numeric. Crucially, ensure that the visible link text accurately matches the actual URL, and avoid excessive redirects or suspicious shortened URLs.
- Implement Email Authentication: Properly implement and maintain SPF, DKIM, and DMARC for all your sending domains to verify legitimacy and reduce the likelihood of your content being flagged as suspicious or spoofed.
- CDN Configuration & Trust: When using a CDN, consider setting up a CNAME record using your organization's domain to enhance trust. Verify the security and reputation of your chosen CDN and its origin servers, as issues there can impact your content.
- Adhere to Google Guidelines: Familiarize yourself with Google Safe Browsing and Search Central guidelines to ensure your web content and email practices do not violate their policies regarding malware, phishing, or deceptive practices.
What email marketers say
13 marketer opinions
Google's advanced security systems often identify CDN or email content as malicious for a variety of reasons, generally involving concerns about trustworthiness and security. A significant contributor is the sender's reputation, or the reputation of domains associated with linked content, which can be negatively impacted by high spam complaints, blocklistings, or a history of abusive practices. Beyond reputation, the content itself is scrutinized for characteristics of malicious intent, such as direct malware, exploit kits, or phishing attempts marked by suspicious links, deceptive phrasing, or misleading visual-text combinations. Specific issues with links, including their length, numeric nature, redirects, or a mismatch between visible text and the actual URL, can also trigger alerts. Finally, compromised infrastructure at the source of CDN assets, or a failure to implement strong email authentication like SPF, DKIM, and DMARC, makes content more susceptible to being flagged as untrustworthy.
Key opinions
- Reputation Issues: A primary reason for Google flagging content is a poor reputation of the sending domain, associated IPs, or any linked domains, often due to high spam complaints, blocklisting, or a history of abuse.
- Malicious Content: Google actively detects and flags actual malicious content, including malware, exploit kits, suspicious JavaScript, or phishing attempts characterized by deceptive links, suspicious phrasing, or misleading text-image combinations, even when served by a CDN.
- Link Characteristics: Specific attributes of links, such as being excessively long or numeric, involving multiple redirects, or having visible text that differs from the actual URL, frequently trigger malicious warnings.
- Compromised Infrastructure: If the domain hosting CDN assets or the underlying website it serves is compromised with malware, shell scripts, or spam, or has a history of serving malicious content, it can lead to all linked content being flagged.
- Weak Authentication: Insufficient or improperly configured email authentication protocols, such as SPF, DKIM, and DMARC, can make emails appear suspicious and contribute to their content being flagged as potentially malicious.
Key considerations
- Prioritize Reputation: Actively monitor and work to maintain a strong sender, domain, and IP reputation, using tools like Google Postmaster Tools, and address any sources of high spam complaints or blocklist entries.
- Secure Hosting & Content: Regularly scan all linked websites, origin servers, and CDN-hosted assets for malware, suspicious code, or vulnerabilities, ensuring that images and other content are hosted securely and pass necessary authentication checks.
- Refine Link Management: Optimize all links within your emails by making tracking URLs shorter and non-numeric where possible, ensuring link text accurately matches the destination URL, and avoiding multiple redirects or overly suspicious shortened links.
- Implement Strong Authentication: Ensure that SPF, DKIM, and DMARC are fully and correctly implemented for all your sending domains to verify your email's legitimacy and reduce its susceptibility to being flagged as spoofed or malicious.
- Evaluate CDN Use: When utilizing CDNs, consider setting up a CNAME to use your own organizational domain for hosted assets, and if persistent flagging occurs, explore hosting images and other content directly through your ESP.
Marketer view
Marketer from Email Geeks explains potential reasons for Google marking content as malicious, including reputation-based issues, creative containing IP addresses, domain mismatches where link text differs from the actual URL, images not hosted by the sending domain (which should pass SPF and DKIM), and bouncing sender or reply-to addresses.
14 Jan 2025 - Email Geeks
Marketer view
Marketer from Email Geeks shares their experience, suggesting that long or numeric tracking links might sometimes trigger malicious warnings and recommends making them shorter or non-numeric.
16 Dec 2022 - Email Geeks
What the experts say
2 expert opinions
Google's primary goal in flagging email or CDN content as malicious is user protection. This is achieved through sophisticated filtering that identifies characteristics of phishing, scams, and other fraudulent activities. Such flags are triggered by elements like suspicious links, deceptive sender information, or any link, even if served via a CDN, that directs users to a site identified as harmful by services like Google Safe Browsing.
Key opinions
- Phishing & Scams Detection: Gmail's filters are designed to detect characteristics commonly associated with phishing or fraudulent activities, such as suspicious links or deceptive sender information, flagging such content as potentially malicious to protect users.
- Safe Browsing for Links: Google's Safe Browsing service is a key mechanism for flagging email content, especially links, when they lead to websites identified as hosting malware or phishing, even if those links point to a CDN.
- Indirect Link Malice: An email can be flagged if a link within it, even one pointing to a CDN, leads to a compromised or disreputable domain, even if the email content itself is not directly malicious.
- Negative Reputation Impact: Being flagged for malicious content, directly or indirectly via links, negatively impacts the sender's reputation, potentially leading to future emails being filtered or blocked.
Key considerations
- Audit for Deceptive Content: Rigorously review your email content for any elements that could be perceived as phishing, scams, or fraudulent, paying close attention to suspicious phrasing, deceptive calls to action, and sender details.
- Secure All Linked Domains & CDNs: Routinely scan and verify the security of all domains linked within your emails, including CDN-hosted assets and their origin servers, to ensure they are free of malware or phishing content and maintain a positive reputation with services like Google Safe Browsing.
- Prioritize Sender Reputation: Understand that flags on linked content, even if the content is indirectly malicious, can severely harm your sender reputation; proactively manage and protect it to ensure ongoing email deliverability.
Expert view
Expert from Spam Resource explains that Gmail flags email content as potentially malicious, such as scams, when it detects characteristics commonly associated with phishing or fraudulent activities. This can include suspicious links or deceptive sender information. The warning serves to protect users from harm, indicating the content is deemed untrustworthy based on Gmail's sophisticated filtering algorithms and reputation checks.
10 Jul 2022 - Spam Resource
Expert view
Expert from Word to the Wise explains that Google flags email content, including links, as malicious through its Safe Browsing service. If a link within an email, even one pointing to a CDN, leads to a site Google has identified as hosting malware or phishing content, that email can be flagged or blocked. This negatively impacts the sender's reputation, potentially leading to future emails being filtered, even if the content itself is not directly malicious, but rather links to a compromised or disreputable domain or CDN.
19 Apr 2022 - Word to the Wise
What the documentation says
5 technical articles
Google may flag CDN or email content as malicious due to its robust user protection systems, which identify actual threats, assess the reputation of associated domains, and detect compromised infrastructure. This comprehensive approach aims to safeguard users from phishing, malware, and deceptive online activities.
Key findings
- Direct Malicious Content: Google's automated systems detect and flag content, including that served via CDNs, if it contains malware, phishing attempts, unwanted software, or other forms of malicious code, such as embedded JavaScript.
- Poor Reputation Scores: A low domain or IP reputation, often resulting from high spam rates, user complaints, or existing blacklistings, can cause Google's filters to flag associated emails and any CDN-linked content as potentially malicious.
- Compromised Origin Servers: Content served through a CDN can be flagged if the origin server, from which the CDN pulls its data, is compromised with malware or hosts malicious files, triggering security warnings.
- Deceptive Practices: Content is flagged if it exhibits patterns associated with deceptive practices or violates Google's webmaster guidelines, aiming to protect users from fraudulent activities.
Key considerations
- Ongoing Reputation Management: Actively monitor and manage your domain and IP reputation using tools like Google Postmaster Tools to identify and mitigate issues such as high spam complaints or blacklistings that can lead to content flagging.
- Secure Content Hosting: Regularly audit and secure your origin servers and all CDN-hosted content, including images and scripts, to ensure they are free from malware, malicious code, or vulnerabilities that Google's systems might detect.
- Adherence to Google Guidelines: Ensure all your web and email content, including CDN-served assets, complies with Google Safe Browsing and Search Central guidelines to avoid being flagged for phishing, malware distribution, or deceptive practices.
- Understand CDN Vulnerabilities: Recognize that while CDNs enhance security, they can still serve compromised content if the origin server is infected; therefore, focus on the security of your source material.
Technical article
Documentation from Google Safe Browsing explains that their service identifies unsafe websites, including those hosting phishing scams, malware, or unwanted software, and warns users. If CDN-hosted content or a linked site falls into these categories, it will be flagged.
10 Aug 2022 - Google Safe Browsing
Technical article
Documentation from Google Postmaster Tools Help explains that a poor domain or IP reputation, often influenced by high spam rates, user complaints, or blacklisting, can cause emails, including any CDN-linked content within them, to be marked as spam or potentially malicious by Google's filters.
1 May 2022 - Google Postmaster Tools
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