How do URLs in newsletters affect email deliverability and what tools can assess URL reputation?
Matthew Whittaker
Co-founder & CTO, Suped
Published 22 Apr 2025
Updated 17 Aug 2025
10 min read
The URLs embedded in your newsletters play a surprisingly significant role in your email deliverability. It's not just about the content of your email, but also about where those links lead and the path they take to get there. Mailbox providers (ISPs) like Gmail and Outlook are constantly evaluating the trustworthiness of every element in an incoming email, and links are a major factor in that assessment. A single problematic URL can send an otherwise perfectly crafted newsletter straight to the spam folder, impacting your sender reputation and ultimately, your campaign's effectiveness.
I've often seen senders overlook the importance of link hygiene, focusing instead on sender score or email content. However, an email's links can carry significant weight, influencing whether it reaches the inbox or is filtered out. Understanding how URL shorteners, redirects, and the reputation of linked domains affect deliverability is crucial for anyone sending newsletters.
The challenge is that the reputation signals from URLs are often hidden. It's not always obvious why one email lands in the inbox and another, seemingly identical, one ends up in spam, especially when using the same sending infrastructure. This guide will explore how URLs impact deliverability and what you can do to maintain a healthy link reputation.
How URLs impact deliverability
When an ISP receives an email, it doesn't just look at the sender's IP and domain reputation. It also scans every link within the email body. Each URL is evaluated based on its own domain's reputation, its history, and whether it's associated with any known spam or phishing activities. If a link points to a low-reputation site, even if your sender reputation is stellar, it can trigger spam filters.
One common pitfall is the use of public URL shorteners (like Bitly). While convenient, these services are often abused by spammers, which can lead to their domains being blocklisted. If you use a shortened link from a service that has a poor reputation, your email is more likely to be flagged, regardless of the ultimate destination. This is why it's generally advised to avoid them in marketing emails or use branded, custom short domains you control.
Another factor is the use of redirects and tracking links. Many email service providers (ESPs) and marketing platforms use their own tracking domains for clicks. While this is standard practice, if the ESP's tracking domain has a poor reputation due to other users' spammy behavior, it can negatively affect your emails. It's always better if the tracking domain is a subdomain of your own sending domain, allowing you to control its reputation. This also applies to HTTP tracking links versus HTTPS, though most providers now use HTTPS by default.
Understanding URL reputation and its importance
URL reputation, often referred to as domain reputation, is a score assigned to a domain by ISPs. This score indicates how trustworthy a domain is, based on its history of sending email, hosting content, and user engagement. A high reputation means that content originating from or linked to that domain is generally considered safe, while a low reputation signals potential threats like spam, malware, or phishing.
ISPs monitor various signals to determine URL reputation, including how long the domain has existed, whether it's associated with known spam traps, and user complaints related to content hosted on that domain. The more external links you include, especially to domains you don't control, the more you implicitly take on their deliverability risk. This is particularly tricky with news sites or affiliate links, which might be legitimate but could also be used by bad actors.
If you are linking to external sites in a newsletter, it's critical that those domains have a good standing. A single problematic URL can cause your entire email to be blocked or sent to spam. This is why direct, unmasked links to untrusted or suspicious domains should be avoided. Prioritizing links to trusted, reputable websites is a key practice for improving your email's deliverability, as they signal reliability to ISPs. Conversely, linking to low-reputation sites can quickly damage your sender reputation.
The good news is that ISPs generally prioritize sender reputation that is tied to the sending domain in the mail. If you own the domain that appears in the From header and use it for your click tracking, the impact of third-party links is significantly reduced. This is because the initial reputation check is done on your owned domain, and only then do ISPs follow the redirect to the final destination. This strategy avoids 95% of potential issues related to external URL reputation.
Tools for assessing URL reputation
Assessing URL reputation can be challenging because most public tools offer only a general safety rating. Tools like Norton Safe Web, McAfee SiteAdvisor, and Google Safe Browsing will tell you if a site is generally safe from malware or phishing, but they don't provide granular data on how a link might affect your email's inbox placement specifically. This is because email deliverability reputation is often proprietary to each ISP.
While there isn't one single magic tool that gives you a definitive deliverability score for a URL, some tools can provide valuable insights into a link's path and potential issues. URLScan.io is excellent for revealing the full redirection path of a URL. This can help identify any intermediate redirects that might be problematic. Similarly, VirusTotal.com scans URLs against multiple antivirus engines and blacklist databases, offering a broader security perspective. These tools won't directly tell you about inboxing, but they can highlight suspicious behavior or known threats.
For a more direct assessment, I'd suggest using an email deliverability testing service or platform. While not directly URL reputation checkers, these services often include checks against various blocklists (or blacklists) and can show you where your email lands across different ISPs, which can indirectly indicate if a URL is causing issues. If you notice a consistent drop in deliverability when a specific URL is present, that's a strong indicator.
Remember that ISPs use a complex algorithm that combines various factors. A URL's reputation is just one piece of the puzzle. Other critical elements include your sender authentication (SPF, DKIM, DMARC), recipient engagement, spam complaint rates, and the overall content quality. It's often a combination of factors, not just one, that leads to deliverability issues.
Best practices for linking in newsletters
To safeguard your deliverability, adhering to certain best practices when including URLs in your newsletters is essential. These practices help build trust with ISPs and ensure your emails reach the intended inbox.
Use branded tracking links: Configure your email platform to use a custom tracking domain that is a subdomain of your main sending domain (e.g., clicks.yourdomain.com). This ties the click reputation directly to your brand, which you control.
Avoid generic URL shorteners: Steer clear of public services unless absolutely necessary, as they carry inherent risks due to potential abuse by spammers. Instead, use full, readable URLs or brand your own short links.
Link to reputable sources: Only link to websites with established, strong reputations. If you're unsure, search for the domain's history or any spam reports associated with it.
Scan your links: Periodically use tools like URLScan.io or VirusTotal to check your outgoing URLs for any red flags, especially for redirects or affiliate links.
Keep URLs clean: Avoid unnecessary parameters or overly complex structures. Unencoded or unusually long URLs can sometimes trigger spam filters.
By proactively managing the URLs in your newsletters, you're adding another layer of defense against spam filters and actively contributing to a stronger sender reputation. It's an often-overlooked aspect of deliverability that can yield significant improvements.
Advanced analysis and final thoughts
Understanding why certain emails inbox and others don't, despite using the same sending infrastructure, is a common frustration for email marketers. It highlights the nuanced nature of email deliverability, where many factors interact. One of these often-underestimated factors is the reputation of the URLs included in the email.
Statistical analysis, such as Media Mix Modeling or multiple regression, can be highly effective in identifying problematic domains. By correlating the presence of specific domains in your newsletters with bounce or click rates, you can pinpoint which URLs might be negatively impacting your deliverability. While these methods require a data science approach, they offer a deeper insight than standard lookup tools.
Ultimately, a holistic approach to email deliverability includes diligent monitoring of your sender reputation, careful content creation, and a strategic approach to link management. Every element within your email contributes to how ISPs perceive it, and proactive management of all these factors is key to consistent inbox placement.
Views from the trenches
Best practices
Always use branded tracking domains (subdomains of your main sending domain) for all links.
Regularly audit all third-party links in your newsletters, including affiliate links, for reputation issues.
Prioritize linking to established, high-reputation websites to maintain trustworthiness with ISPs.
Implement a consistent process for reviewing and verifying the integrity of all URLs before each send.
Monitor your engagement metrics closely; sudden drops could indicate a URL-related deliverability issue.
Common pitfalls
Using generic, public URL shorteners that are frequently abused by spammers and have poor reputations.
Not owning the tracking domain used by your ESP, which means you can't control its reputation.
Linking to untrusted or unknown domains without prior verification, which can trigger spam filters.
Ignoring the full redirect path of a URL, as intermediate redirects can also carry reputation risks.
Failing to correlate specific URLs in newsletters with deliverability performance metrics like bounce rates.
Expert tips
Consider how often email providers update their blacklist or blocklist information and how quickly they react to new threats.
Remember that each domain you link to has its own reputation, and ISPs will factor this into their decisions.
Even legitimate news sites can be problematic if spammers also use them, making it hard to pinpoint issues.
Statistical modeling of newsletter data can help identify problematic domains by correlating their presence with poor deliverability.
Focus on the initial domain in the email (your own) if it redirects, as it mitigates much of the external link risk.
Marketer view
Marketer from Email Geeks says they use URLScan.io because it gives the full redirection path, which is helpful for identifying issues.
2023-11-09 - Email Geeks
Expert view
Expert from Email Geeks says that mailbox providers are the only ones who truly know a URL's reputation, so it's important to look for domains that bad actors might also use, especially with affiliate programs.
2023-11-09 - Email Geeks
Navigating the complexities of URL reputation
URLs are more than just destinations; they are critical signals that ISPs use to assess the trustworthiness of your emails. Neglecting the reputation of the links within your newsletters can severely hinder your deliverability, regardless of other positive sender attributes. By adopting best practices like using branded tracking domains, avoiding generic shorteners, and linking to reputable sources, you can significantly improve your email's chances of reaching the inbox.
While public tools offer some insight into general URL safety, the most effective strategy involves combining these with a deep understanding of your own sending practices and leveraging analytical methods to identify specific problematic links. Proactive link management is a cornerstone of effective email deliverability, ensuring your newsletters land where they belong: in your subscribers' inboxes.
Gmail and 
Outlook are constantly evaluating the trustworthiness of every element in an incoming email, and links are a major factor in that assessment. A single problematic URL can send an otherwise perfectly crafted newsletter straight to the spam folder, impacting your sender reputation and ultimately, your campaign's effectiveness.
Bitly). While convenient, these services are often abused by spammers, which can lead to their domains being blocklisted. If you use a shortened link from a service that has a poor reputation, your email is more likely to be flagged, regardless of the ultimate destination. This is why it's generally advised to avoid them in marketing emails or use branded, custom short domains you control.
