How do security filters evaluate link reputation?
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You've carefully written your email, built a clean template, and authenticated your domain. Then a link inside your email gets flagged by a security filter and your message never reaches the inbox. What actually happened?
Security filters scrutinize every link in an email because URLs are the most common delivery vehicle for phishing, malware, and credential theft. When a message arrives, the filter doesn't just glance at the link text. It runs the URL through several layers of checks before deciding whether to let it through.
The first check is reputation databases. Filters query services that maintain lists of known malicious destinations. Google Safe Browsing, PhishTank, and VirusTotal are widely used examples. If your URL matches a flagged domain, the filter blocks it immediately.
The second check is destination crawling. Smarter filters don't just look up a URL. They follow it. The filter visits the landing page, scans for phishing forms, checks for malware payloads, and traces redirect chains all the way to the final destination. This is how filters catch brand-new threats that haven't made it into reputation databases yet.
The third check is domain signals. Even if a URL isn't on any blocklist, the domain itself raises or lowers suspicion. Newly registered domains are treated with a lot of caution. Domains with a spotty history get flagged. Domains that look almost like a well-known brand (think paypa1.com or arnazon.com) get caught as typosquatting attempts.
URL shorteners and tracking links deserve their own mention. Shorteners obscure the real destination, which makes filters suspicious by design. Many enterprise gateways will either block shortened links outright or expand them before deciding. Tracking links (like the wrapped click-tracking URLs your ESP generates) are different. They're usually tied to a known sending domain, which is why your domain's overall reputation matters so much. If your authentication is solid and your sending domain is clean, those tracking links get less scrutiny. If your domain has a weak reputation, every link inside your email gets a harder look.
The practical takeaway is straightforward. Use your own branded tracking domain rather than a shared one from your ESP. Avoid URL shorteners in marketing emails. Don't link to newly registered domains. And if you're sharing files, link to cloud storage (Google Drive, Dropbox) rather than embedding direct download URLs, since security gateways are particularly suspicious of those.
But if you're seeing unexpected delivery failures on emails with links, our free Email Header Analyzer can sometimes surface which filter flagged the message and why. For a harder-to-diagnose situation, our SOS hotline is free and we actually pick up.
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