Why do email filters modify or break links?

Key findings

• Security priority: Email filters prioritize security, often rewriting or breaking links to neutralize potential threats like phishing and malware before they reach the recipient's inbox.
• Enterprise environments: Corporate and institutional networks, especially in sectors like healthcare, commonly employ robust email security appliances that are known to modify or even break links due to heightened risk profiles.
• URL rewriting: A common practice is URL rewriting, where the original link is replaced with a proxy URL, allowing the filter to scan the destination page for threats in real time if the link is clicked.
• Partial modification: Often, only a subset of links within an email is affected, typically those deemed suspicious or those that use deceptive formatting to obscure their true destination.
• Compliance implications: Broken links, particularly for critical functions like unsubscribing, can lead to compliance issues and negative user experiences, even if they worked perfectly upon sending.

Key considerations

• Proactive testing: Routinely test your emails across various email clients and security configurations to identify potential link modification issues.
• URL structure: Avoid deceptive URL structures, such as embedding well-known domains within longer, less reputable ones, as these are common triggers for link filters.
• Sender reputation: A strong sender reputation reduces the likelihood of filters treating your links with suspicion. Maintain consistent sending practices and monitor your domain and IP reputation.
• Filter awareness: Recognize that email filtering is a multi-layered process, with various techniques, including link analysis, employed to ensure security. More information on these techniques can be found at Cynet's guide to email filtering techniques.

Key opinions

• Common occurrence: It is widely acknowledged that email filters commonly modify or break links, especially within business environments that deploy advanced security appliances.
• High-risk sectors: Marketers frequently observe this behavior in industries with high security concerns, such as healthcare (hospitals), where the risk of phishing and malware is substantial.
• Compliance headaches: Broken unsubscribe links, a direct result of URL modification by filters like Proofpoint or Microsoft O365, can lead to difficult discussions with regulators regarding email compliance.
• Image links: Links embedded within images are also susceptible to blocking by spam filters if they are perceived as malicious or suspicious content.

Key considerations

• Link type matters: While some filters modify all links, often it's a subset that triggers the filter, such as those with suspicious redirect patterns or cloaking attempts.
• URL shortening services: Be mindful that some link shortening services can also contribute to deliverability challenges, as they can sometimes be flagged by spam filters.
• Excessive hyperlinks: Using too many hyperlinks within an email can flag it as spam. Maintaining a good IP and domain reputation is essential to avoid such issues, as detailed in Spiralytics' guide to avoiding spam filters.
• Bot clicks: Automated security bots often click all links within emails upon delivery, which can trigger URL modification and skew engagement metrics. Understanding how to identify these artificial clicks is important.

Key opinions

• Inevitable modification: Experts confirm that filters, especially in business and high-security settings, absolutely modify and can break links as a standard protective measure.
• Risk mitigation: The primary driver for such aggressive link handling is the severe risk posed by phishing and malware, making it a necessary evil for network security.
• Deceptive URLs: Filters are designed to detect and counter attempts to confuse recipients by presenting misleading URLs, even if the original intent was benign.
• Multi-layered defense: Email filtering involves multiple layers of analysis, where links are scrutinized for categorization, reputation, and potential threats.

Key considerations

• Security vs. deliverability: Recognize the trade-off between strict security protocols and seamless deliverability. Filters prioritize safety, which can sometimes impact the functionality of legitimate links.
• Sophisticated detection: Modern filters use advanced techniques to identify and neutralize phishing attempts, including link analysis that goes beyond simple blocklists. For instance, see this article on bypassing phishing link filters.
• Domain reputation impact: The reputation of your domain plays a critical role in how your links are perceived and handled by filters. A poor reputation can lead to increased scrutiny.
• Technical alignment: Ensuring proper DMARC, SPF, and DKIM alignment is foundational. Authentication failures can prompt filters to treat your links with heightened suspicion, leading to modification or rejection.

Key findings

• Integrated security: Email filtering is presented as an integrated, multi-layered process, with link analysis and modification being a critical step in protecting recipients.
• Threat detection: Filters primarily target malicious links associated with spam, phishing, and malware, using various techniques including header analysis and content scanning.
• Domain blacklisting: Links to uncategorized, malicious, or newly registered domains are frequently blocked or removed by filters as a protective measure.
• URL inspection: Email security systems perform deep inspection of URLs, often rewriting them to safe links or proxies to scan for threats dynamically at the time of click.

Key considerations

• Comprehensive analysis: Email protection involves analyzing messages multiple times before they reach the inbox, which directly impacts how links are handled. More details can be found in the University of Washington's guide on email filtering.
• Header and content filters: Beyond links, filter techniques also include examining email headers and content for spam indicators, which can indirectly affect how links are perceived. For example, hexadecimal sequence errors can trigger filters.
• Trust and reputation: Email filtering heavily relies on the overall reputation of the sender and the domains linked within the email. Emails from unindexed or unknown domains are more likely to have their links flagged.
• Hidden links: Be aware that hidden links or those using advanced tracking mechanisms can also be subject to increased scrutiny and modification by filters, even from legitimate sending sources.