It can be frustrating to send an email with carefully crafted links, only to have recipients report that they are broken, modified, or lead to unexpected destinations. This isn't usually a mistake on the sender's part, but rather an intentional action by email filters and security systems. These systems play a crucial role in safeguarding inboxes, and part of their defense strategy involves inspecting and, at times, altering links within incoming messages.
The primary reason for this behavior is to protect users from malicious content such as phishing scams, malware, and other cyber threats. Email service providers (ESPs) and corporate security appliances are constantly evolving their methods to detect and neutralize these dangers. Modifying links allows them to vet content before it reaches the end user, acting as a critical barrier against potential harm.
While this protective measure is essential, it can inadvertently impact legitimate emails, leading to confusion and deliverability challenges. Understanding why and how these modifications occur is key to improving email deliverability and ensuring your messages reach their intended audience with all links intact and functional.
The purpose of link modification
Email filters modify links primarily for security purposes. The internet is rife with phishing attempts, malware distribution, and other cybercrimes that often use deceptive links. These filters act as a proactive defense mechanism, aiming to identify and neutralize threats before they can compromise a user's device or data.
By rewriting or inspecting links, email filters can route them through their own security systems. This allows them to scan the linked content for malicious code, suspicious redirects, or known phishing indicators. This process is often invisible to the recipient, happening in the background before the email even lands in the inbox.
Some filters even simulate clicks on links in a sandboxed environment to observe their behavior without risking the user. This helps identify links that might appear benign but are designed to lead to harmful sites or download malicious files. It's a robust security layer that, while sometimes inconvenient for senders, is vital for recipient safety.
Why filters modify links
Phishing prevention: Rewriting URLs to redirect through a secure gateway helps identify and block malicious sites before they can harm users.
Malware protection: Links are scanned for malware and viruses, preventing infected files from reaching the recipient's system.
Reputation checks: Filters check link domains against known blocklists and databases of suspicious URLs.
Click tracking: Email Service Providers (ESPs) often modify links for their own analytics, to track opens and clicks, but this is different from security-driven modifications.
How filters modify links
The most common way email filters modify links is through a process called URL rewriting or wrapping. This involves changing the original link in your email to a new URL that redirects through the filter's own servers. When a recipient clicks the link, they first go to the filter's server, which then performs its checks before redirecting them to the intended destination.
For example, a link to `yourwebsite.com/page` might become `safelinks.protection.outlook.com/?url=yourwebsite.com/page`. This new, wrapped URL allows the filter to analyze the target page in real time or check against its blacklists (or blocklists) for malicious content before allowing the user to proceed. If the link is deemed unsafe, the user might see a warning page instead of the original destination.
Another form of modification is when filters automatically click links in a simulated environment to test their behavior. While this doesn't visually change the link for the recipient, it can significantly skew click tracking statistics, making it appear as if users are clicking links when they haven't. This is particularly relevant for hidden links or those used for tracking email opens.
Original link
Appearance: Displays as the sender intended, e.g., https://www.example.com/product.
Direct path: Clicks go directly to the destination website.
Risk: No intermediate security checks by the recipient's mail system.
Modified (rewritten) link
Appearance: The visible link might remain the same, but the underlying href attribute is changed, e.g., safelinks.protection.outlook.com/...example.com.
Redirected path: Clicks first go through the filter's server for scanning.
Enhanced security: Provides a layer of protection against phishing and malware.
Example of a rewritten URL by an email filter
Original URL: https://www.yourdomain.com/landing-page
Modified URL (example): https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.yourdomain.com%2Flanding-page&data=05%7C01%7Cuser%40example.com%7C...&sdata=...&reserved=0
Impact on legitimate emails
While link modification aims to protect recipients, it can inadvertently cause issues for legitimate senders. One significant impact is on email authentication protocols like DKIM. If a filter modifies the email's body, including its links, it can break the DKIM signature. This might lead to DKIM authentication failures if the signing domain doesn't match the new, rewritten URL, impacting your overall sender reputation and deliverability.
When filters rewrite URLs, they can sometimes break the links entirely, especially if the original URL contained complex parameters or non-standard characters. Recipients might click on a link only to be met with an error page, leading to a poor user experience and potential frustration. This is particularly problematic for critical links, such as unsubscribe links required by regulations like CAN-SPAM.
The impact extends to data accuracy, as well. Email marketers rely on click-through rates to gauge engagement and optimize campaigns. When filters automatically click links for security checks, these false clicks skew metrics, making it difficult to get an accurate picture of user behavior and campaign performance.
Impact on deliverability
Description
DKIM alignment failures
When links are rewritten, the email content changes, which can invalidate the DKIM signature, leading to authentication failures.
Broken links
Complex or malformed URLs (e.g., due to unencoded characters) may break during the rewriting process.
Skewed analytics
Automated link scanning by security filters generates false clicks, inflating click-through rates and misrepresenting user engagement.
Recipient frustration
Users may encounter security warnings or broken pages, leading to a negative perception of your emails.
Mitigating the effects of link modification
While you can't prevent email filters from modifying links, you can take steps to minimize negative impacts. Ensure your email authentication protocols, particularly DKIM, are correctly configured. Some filters might bypass rewriting for domains with strong authentication, or they may have mechanisms to re-sign emails after modification.
Using clear, simple, and well-formed URLs can also help. Avoid excessively long or complex links if possible. Always use HTTPS for your links, as this indicates a secure connection and can positively influence how filters perceive your URLs. HTTP links are often flagged as suspicious.
Monitor your domain and IP reputation consistently. A strong reputation can signal to filters that your emails are trustworthy, reducing the likelihood of overly aggressive link modifications or outright blocking. If your domain has a questionable history, filters are more likely to scrutinize your links.
Lastly, consider using a dedicated tracking domain for your email links if your ESP offers it. This can help prevent your main sending domain from being associated with click tracking services, which sometimes look suspicious to filters. A dedicated tracking domain also provides more control over how your links appear and behave.
Views from the trenches
Best practices
Maintain strong sender authentication (SPF, DKIM, DMARC) to build trust with mail servers, which can reduce the need for aggressive link modifications.
Use clear, well-structured HTTPS links in your emails, avoiding unnecessary complexity or parameters that could trigger filter alerts.
Regularly monitor your email deliverability and click-through rates to detect any anomalies that might indicate link modification issues.
Segment your audience and tailor content to minimize the risk of being flagged by specific corporate or industry-specific filters.
Common pitfalls
Relying on old authentication standards can increase the likelihood of filters modifying your links, as they may see your emails as less trustworthy.
Using HTTP instead of HTTPS links makes your emails more susceptible to being flagged or rewritten by security filters looking for secure connections.
Ignoring reports of broken or modified links from recipients, which indicates underlying issues with how filters are processing your emails.
Having a poor sender reputation can lead to more aggressive link scanning and modification, even for legitimate emails.
Expert tips
Ensure your unsubscribe links are always functional and clearly visible, as broken unsubscribe links are a major red flag for both recipients and filters.
Regularly test your email campaigns across various email clients and corporate environments to catch unexpected link modifications.
Be aware that certain industries, like healthcare or finance, often employ stricter email security measures, leading to more aggressive link filtering.
Educate your recipients that link rewriting is a common security practice and that legitimate links may appear different in their email client.
Expert view
Expert from Email Geeks says that filters frequently modify links, especially those deployed by businesses using specialized appliances.
2019-12-05 - Email Geeks
Expert view
Expert from Email Geeks indicates that filters in environments like hospitals will definitely modify and potentially break links due to the high risks of phishing and malware.
2019-12-05 - Email Geeks
Navigating the complexities of email links
Email filters modify or break links primarily for robust security reasons, aiming to protect recipients from phishing and malware. This involves practices like URL rewriting and simulated clicks. While these measures are crucial for cybersecurity, they can inadvertently impact legitimate email deliverability, skew analytics, and occasionally lead to broken links for end-users.
Understanding these mechanisms allows senders to implement best practices, such as strong email authentication and using HTTPS links, to minimize negative effects. By adapting to the evolving landscape of email security, you can enhance your deliverability and ensure your messages reach the inbox with integrity, maintaining trust with your audience.
safelinks.protection.outlook.com/...example.com.
