Summary
Email filters frequently modify or break links for several critical reasons, primarily driven by security concerns and email marketing analytics. Security filters, such as those from Microsoft Defender, Proofpoint, Google Workspace, and Mimecast, rewrite URLs to protect users from phishing, malware, and other cyber threats. This process often involves redirecting clicks through their servers for real-time analysis, known as 'time-of-click verification' or 'URL detonation,' which scans the destination for malicious content before the user accesses it. Beyond security, Email Service Providers (ESPs) like SendGrid and Mailgun modify links for tracking purposes, allowing senders to gather valuable click-through data and understand engagement. While beneficial, these modifications can occasionally lead to legitimate links, including vital unsubscribe links, being inadvertently broken, particularly in highly secure corporate or government environments. This widespread practice is a fundamental part of modern email infrastructure, serving both protective and analytical functions.
Key findings
- Security Against Threats: The primary reason email filters modify links is to protect users from phishing, malware, and other malicious content by rewriting URLs to be scanned or verified in real-time upon click, often through a proxy or sandbox environment.
- Time-of-Click Verification: Services like Microsoft Defender for Office 365 Safe Links, Proofpoint URL Defense, Mimecast URL Protect, and Barracuda employ time-of-click verification, redirecting clicks through their servers for real-time threat analysis.
- Email Service Provider Tracking: Email Service Providers (ESPs) such as SendGrid, Postmark, and Mailgun modify links to enable click tracking and analytics, providing senders with valuable data on email engagement and campaign performance.
- Potential for Breakage: While intended for security or tracking, these modifications can sometimes lead to legitimate links, including crucial unsubscribe links, being broken or rendered non-functional, causing deliverability and compliance issues.
- Widespread Practice: Link modification is a common practice across a wide range of email security solutions, ISPs, and ESPs, including major players like Google Workspace, Microsoft, Proofpoint, Sophos, and Mimecast.
Key considerations
- Test All Links: Always thoroughly test all links, especially critical ones like unsubscribe links, to ensure they function correctly after potential modification by various filters.
- Understand Tracking Impact: Recognize that email service providers rewrite links for tracking, which means the URL recipients see in their inbox is often not the original, and understand how this impacts your analytics.
- Corporate Email Environments: Be aware that corporate and government email systems, like those in hospitals, often have very aggressive security filters that are more prone to modifying or breaking links due to heightened security protocols.
- Well-Formed URLs: Ensure your URLs are always correctly formatted and follow best practices to minimize the risk of legitimate links being misinterpreted or broken by stringent security or spam filters.
What email marketers say
8 marketer opinions
Understanding why email filters alter links reveals a dual purpose: safeguarding recipients and empowering senders with crucial engagement data. Both email service providers (ESPs) and various security solutions routinely rewrite URLs. This common practice ensures protection against malicious content like phishing and malware, often by routing clicks through proxy servers for real-time analysis. Simultaneously, ESPs implement link modifications to track user interactions, providing valuable insights into campaign performance. While largely beneficial, these alterations can occasionally lead to legitimate links being broken or rendered non-functional, particularly in highly secure or complex email environments.
Key opinions
- Security Redirection: Email filters frequently rewrite original URLs, directing clicks through security systems to scan for phishing, malware, and other threats in real time.
- Engagement Tracking: Email Service Providers (ESPs) modify links to unique tracking URLs, allowing senders to monitor metrics such as click-through rates and gain insights into email campaign performance.
- Corporate Defense Layer: Corporate email security solutions specifically rewrite links to act as a proxy for real-time analysis, adding a vital layer of defense against malicious URLs for employees.
- Purposeful Modification: The core reasons for link modification are to enhance user security and provide comprehensive analytics for email marketing efforts.
- Potential for Inadvertent Breakage: Despite their intended benefits, these link modifications can sometimes inadvertently disrupt or break legitimate URLs, especially if the original link structure is unusual or in highly restrictive security setups.
Key considerations
- Anticipate URL Changes: Senders should expect that their original URLs will be rewritten by both ESPs for tracking and by various security filters.
- Verify Link Functionality: It is crucial to thoroughly test all links within emails across different clients and environments to ensure they remain functional after modification.
- Security System Impact: Be aware that stringent corporate and ISP security systems may heavily modify or even break links, requiring careful testing and understanding of potential recipient experiences.
- Optimize for Deliverability: Design emails with well-formed, standard URLs to minimize the chances of legitimate links being misinterpreted or negatively impacted by aggressive filtering.
Marketer view
Email marketer from Postmark Blog explains that email service providers often modify links within emails for tracking purposes. This involves rewriting original URLs to unique tracking URLs that redirect through the ESP's servers, allowing them to record metrics like click-through rates and provide analytics to senders.
2 Oct 2022 - Postmark Blog
Marketer view
Email marketer from Validity Blog explains that email filters, including those operated by Internet Service Providers (ISPs), often modify links to protect users from phishing and malware, and for senders to track email engagement. These modifications usually involve rewriting the original URL to route clicks through security scanning or analytics platforms.
19 May 2025 - Validity Blog
What the experts say
5 expert opinions
Email filters routinely modify or even break links within messages, primarily as a critical security measure to protect recipients from various online threats. This practice is particularly prevalent in business and institutional environments, such as hospitals, where advanced security products like Microsoft's Safe Links and Proofpoint actively rewrite URLs. Their purpose is to inspect the link's destination for malicious content, including phishing attempts and malware, before a user can inadvertently click. Furthermore, filters may selectively target and alter links that appear deceptive, even if from a legitimate source. While safeguarding users is the main objective, these modifications can unfortunately result in broken links, impacting even essential functionalities like unsubscribe options.
Key opinions
- Primary Security Measure: Email filters modify links mainly as a crucial security measure, scanning for phishing, malware, and other threats by inspecting link destinations before user interaction.
- Corporate Environment Focus: Business and institutional settings, such as hospitals, employ stringent security filters that commonly rewrite URLs to mitigate risks from phishing and malware.
- Targeted Link Modification: Filters can selectively alter a subset of links, especially those perceived as deceptive or similar to phishing attempts, even if they originate from legitimate sources.
- Leading Service Practices: Prominent services like Proofpoint and Microsoft 365 (O365) are known to modify email URLs, a practice that can unfortunately result in broken links.
- Unintended Link Breakage: Despite their protective intent, link modifications by filters can inadvertently lead to legitimate links, including critical unsubscribe options, becoming non-functional.
Key considerations
- Anticipate Security Rewrites: Senders should always expect that email filters, especially in corporate and institutional environments, will rewrite or modify their links for security purposes.
- Verify Unsubscribe Links: It is critical to meticulously test all links, particularly the unsubscribe link, to confirm they remain functional even after being modified by filters, preventing compliance issues.
- Impact on User Experience: Recognize that aggressive security modifications, while protective, can inadvertently lead to a negative user experience by rendering legitimate links, like calls to action, non-functional.
- Understand Filter Logic: Be aware that filters may selectively target and modify links that appear deceptive or similar to phishing attempts, regardless of the sender's legitimacy.
Expert view
Expert from Email Geeks explains that email links are often modified or broken by filters, especially in business environments like hospitals, where security filters aim to mitigate risks from phishing and malware.
3 Sep 2022 - Email Geeks
Expert view
Expert from Email Geeks shares insights on how email filters can selectively modify a subset of links, particularly those that might appear deceptive or similar to phishing attempts, even if from a legitimate source.
5 Mar 2025 - Email Geeks
What the documentation says
6 technical articles
Email filters, particularly those integrated into advanced security solutions, commonly modify or break links within messages as a proactive measure against cyber threats. This widespread practice, employed by leading providers like Microsoft, Google, Proofpoint, Mimecast, Barracuda, and Sophos, primarily involves rewriting URLs. The rewritten links redirect clicks through the security vendor's servers or cloud infrastructure for real-time analysis, often termed "time-of-click verification" or "URL detonation." This crucial step allows the filter to scan for phishing attempts, malware, and other malicious content before the user accesses the original destination, thereby preventing harm. While designed for robust protection, these modifications, though essential, can sometimes inadvertently affect legitimate links.
Key findings
- Proactive Threat Defense: Email filters modify links primarily to provide time-of-click protection against phishing, malware, and other advanced cyber threats, acting as a crucial preventative layer.
- URL Rewriting Mechanism: The core method involves rewriting original URLs to redirect user clicks through the security provider's systems for real-time scanning and verification.
- Real-time Analysis: This modification enables immediate analysis of the link's destination, using threat intelligence, sandboxing, or reputation checks to ensure safety before the user reaches the intended site.
- Industry Standard Practice: Leading email security providers, including Microsoft, Google, Proofpoint, Mimecast, Barracuda, and Sophos, all implement some form of URL modification for security purposes.
- Prevention of Malicious Access: The ultimate goal of link modification is to prevent users from inadvertently accessing dangerous or compromised websites.
Key considerations
- Expect Link Transformation: Senders should anticipate that email links will be transformed by various security filters, making the displayed URL different from the original.
- Verify Post-Modification Functionality: Always thoroughly test all links in emails to ensure they remain functional and lead to the correct destination after potential modification by security filters.
- Understand Security Logic: Be aware that filters use sophisticated logic, including real-time checks and sandboxing, to evaluate link safety, impacting how links behave for recipients.
- Impact on Reporting: Recognize that security filter redirects might affect how link clicks are attributed or reported in your analytics, as the immediate click often goes to the security vendor first.
Technical article
Documentation from Microsoft Learn explains that email filters, specifically Safe Links in Microsoft Defender for Office 365, modify URLs in incoming emails to provide time-of-click verification. This rewriting helps protect users from malicious links by redirecting them through Microsoft's servers, where the URL is checked for known bad reputation or suspicious content before the user accesses the original destination.
30 Sep 2021 - Microsoft Learn
Technical article
Documentation from Google Workspace Admin Help explains that email filters in Gmail modify links as part of their advanced phishing and malware protection. This involves scanning links for malicious content and, in some cases, rewriting them to redirect through Google's safe browsing services to prevent users from accessing dangerous websites.
21 Jan 2025 - Google Workspace Admin Help
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