Why are unencoded URLs problematic for email deliverability?

Unencoded URLs can confuse spam filters, leading them to misinterpret the link's legitimacy. Special characters, like spaces, in a URL have a specific encoded format ( %20 ). If not encoded, the URL appears malformed, raising suspicion and increasing the email's spam score, diverting it to junk or spam.

How do spam filters identify unencoded URLs?

Spam filters analyze email content for anomalies. Unencoded URLs can look like attempts to obfuscate malicious links or could be misinterpreted as security threats, such as SQL injection attempts, especially if they contain common command words like 'update' or 'select'.

What characters commonly need to be URL encoded?

Common characters that require URL encoding include spaces ( %20 ), parentheses like '(' ( %28 ) and ')' ( %29 ), and other symbols such as '&' ( %26 ), and '+' ( %2B ). These are commonly found in query parameters or dynamic content.

Can unencoded URLs affect link tracking and analytics?

Yes, unencoded URLs can significantly impact link tracking. If the URL is malformed, the tracking redirect might break, preventing your ESP from logging clicks or correctly passing parameters to your analytics tools, leading to inaccurate campaign data.

How can I check if my email URLs are properly encoded?

You can test your emails using an email deliverability tester that checks how your links render and behave across different email clients and spam filters. Manually inspect your URL parameters to ensure they are properly encoded before sending.

How do unencoded URLs impact email deliverability and spam filtering? - Technical - Email deliverability - Knowledge base

Email deliverability is a complex landscape where even small technical missteps can significantly impact whether your messages reach the inbox or are diverted to the spam folder. One such overlooked detail is the proper encoding of URLs within your emails.

URLs, or Uniform Resource Locators, are the addresses that direct recipients to specific web pages. They often contain various characters, some of which have special meanings in web contexts. To ensure these characters are interpreted correctly and don't break the URL, they must be URL encoded. This process converts problematic characters (like spaces or certain symbols) into a format that web browsers and mail transfer agents can universally understand.

When URLs are not properly encoded, particularly in dynamic elements like UTM parameters, they can introduce errors that affect both the user experience and how spam filters perceive your email. While modern web browsers are often forgiving and can correct these malformations on the fly, email clients and spam filters operate with a much stricter interpretation.

The consequences can range from broken links to your emails being flagged as suspicious, ultimately impacting your sender reputation and deliverability rates. Understanding the nuances of URL encoding is crucial for any sender aiming for optimal inbox placement.

Understanding URL encoding

URL encoding is the process of converting characters in a URL that are not allowed or have special meaning into a percent-encoded format. For example, a space character (' ') is encoded as %20, while other characters like '(', ')', or '*' also have their corresponding encoded forms. This standardization ensures that URLs are transmitted across the internet without ambiguity or corruption.

When you use UTM parameters or other query strings in your email links, they often contain dynamic values such as campaign names with spaces or special characters. If your email service provider (ESP) or marketing platform doesn't automatically encode these parameters, you end up with unencoded URLs.

Here's a quick comparison of an unencoded versus an encoded URL:

Aspect	Unencoded URL	Encoded URL
Example	http://example.com/campaign?name=My Campaign (Q1)	http://example.com/campaign?name=My%20Campaign%20%28Q1%29
Readability	Looks more human-readable, but technically problematic.	Less human-readable, but technically correct and safe.
Compatibility	May cause issues with some mail clients or spam filters.	Works universally across all platforms and systems.

While most modern browsers are designed to be resilient and can often auto-correct improperly formatted URLs, email systems are much less forgiving. This discrepancy often leads to deliverability problems that are hard to diagnose if you're only testing your links in a browser.

How unencoded URLs affect spam filters and deliverability

Spam filters (also known as content filters or anti-spam engines) are sophisticated algorithms designed to identify and block suspicious email content. Unencoded URLs can raise red flags for these systems for several reasons. Historically, crappy URLs were a sign of spamware, and while heuristics evolve, the underlying principle of scrutinizing malformed content remains.

When a URL contains unencoded spaces or special characters, it can appear malformed or even intentionally obfuscated to a spam filter. This can contribute to a higher spam score, increasing the likelihood that your email is quarantined or sent directly to the junk folder. Major mailbox providers (MBPs) like

Google and

Yahoo, as well as corporate filters, might be quick to flag such anomalies.

Furthermore, an unencoded URL can also be misinterpreted by spam filters as an attempt at something malicious like a SQL injection attack. If your URL parameters contain words or patterns that resemble database commands (e.g., 'UPDATE', 'DELETE', 'SELECT') and they are not properly encoded, filters may view this as a security risk and block your email. This kind of advanced email content filtering is designed to protect recipients from phishing and other cyber threats.

Potential deliverability problems

Increased spam score: Unencoded characters can trigger spam filters, pushing emails into junk folders.
Broken links: Recipients may encounter 404 errors or incorrect landing pages, damaging the user experience.
Reputation damage: Consistent issues can lead to lower sender reputation and more pervasive blocklisting (or blacklisting).

User experience and technical issues

Beyond the technical parsing by spam filters, unencoded URLs can severely degrade the recipient's experience. While browsers are designed to be tolerant of malformed URLs, email clients are often not. This means a link that works perfectly fine when pasted into a browser might break when clicked from an email.

A common scenario involves tracking URLs from ESPs. Your ESP's redirect link might be valid, but if the final destination URL or its parameters are not fully encoded, the recipient's browser (after the redirect) might barf or display an error page. This can frustrate users and lead to unsubscribes or complaints, further harming your sender reputation.

Furthermore, the length of URLs and the presence of special characters (like parentheses or symbols) can also interact negatively with email clients. Some clients, especially older desktop versions, might not handle these correctly, leading to display issues or even security warnings. This is also related to malformed HTML within the email itself, creating a compounded problem.

It's important to remember that every broken link or unexpected redirection erodes trust with your subscribers. Ensuring all URLs are correctly encoded is a fundamental step in maintaining a positive user experience and safeguarding your email program's integrity.

Best practices for URL handling

To prevent unencoded URLs from impacting your deliverability and spam filtering, proactive measures are key. The first step is to communicate with your email service provider (ESP) or marketing automation platform. They should be responsible for automatically encoding any parameters they append to your tracking URLs.

If you're manually constructing URLs or have custom parameters, ensure you use proper URL encoding functions in your programming language or platform. For example, in many environments, you can use functions like encodeURIComponent() in JavaScript or urlencode() in PHP.

JavaScript example for URL encodingjavascript

const originalURL = "https://example.com/page?param=value with spaces";
const encodedURL = encodeURIComponent(originalURL);

console.log(encodedURL);

Consider adopting a naming convention for your campaign parameters that avoids spaces and problematic special characters altogether. Using underscores or hyphens instead of spaces for elements like campaign names can circumvent many encoding issues, even if your platform is supposed to handle it. This also ties into URL alignment for redirects, where consistency across the entire URL path is beneficial.

Finally, test your emails thoroughly before sending, not just in a browser, but using actual email clients across different devices. This helps you catch any rendering or functionality issues related to unencoded URLs before they reach your subscribers.

Views from the trenches

Best practices

Ensure your ESP automatically URL-encodes all dynamic parameters appended to your tracking links.

Adopt consistent naming schemes for UTM parameters that avoid spaces and special characters, using hyphens or underscores.

Regularly test email links across various email clients and devices to identify any encoding-related breakage early.

Common pitfalls

Assuming that what works in a web browser will work identically in all email clients or with spam filters.

Overlooking unencoded spaces or special characters in dynamic UTM parameters or custom tracking fields.

Not considering that certain words in unencoded URLs might trigger SQL injection heuristics in advanced spam filters.

Expert tips

If you encounter a landing page error after clicking an email link, check the redirected URL for unencoded characters.

Work with your web development team to ensure your site's backend is robust against potential URL parsing issues.

Be mindful of any marketing information, such as internal segment names, inadvertently exposed through unencoded URLs.

Expert view

Expert from Email Geeks says crappy URLs used to be a sign of spamware, so it's not surprising if some filters still notice them, especially if the link won't load.

2019-12-19 - Email Geeks

Marketer view

Marketer from Email Geeks says that while mailbox providers might not care too much, some desktop clients, like Microsoft Outlook, can be very sensitive to URLs and quickly flag them as scam or spam.

2019-12-19 - Email Geeks

Ensure your links are inbox-ready

Proper URL encoding is a seemingly minor technical detail that holds significant weight in the world of email deliverability and spam filtering. While web browsers might compensate for unencoded characters, email systems and security filters are far less forgiving.

Unencoded URLs can lead to broken links, a poor user experience, and, most importantly, can trigger spam filters, causing your emails to be blocklisted (or blacklisted) or land in the junk folder. This directly impacts your campaign performance and sender reputation.

By ensuring your ESP handles encoding correctly, adopting clean naming conventions for your URL parameters, and thoroughly testing your email links, you can avoid these pitfalls. Prioritizing correct URL hygiene is a fundamental step in ensuring your emails consistently reach their intended recipients and provide a seamless experience.