How does URL length affect email deliverability?

Key findings

• Direct Impact Minimised: Modern email systems are generally robust enough to handle long URLs without direct deliverability penalties based on length. The perceived issue of long URLs being a significant problem is largely a myth in contemporary email environments.
• Indirect Impact on Clipping: Excessively long URLs can contribute to the overall HTML size of an email, potentially causing email clipping in clients like Gmail. Clipped emails might hide important content, such as unsubscribe footers, which can negatively affect user experience and compliance.
• Historical Context: In the early 2000s, some older MTA or milter filters might have had vulnerabilities or processing limits that made very long URLs problematic, potentially leading to buffer overflow issues. These are largely resolved now.
• Line Length Limits: RFCs (Request for Comments) define a hard limit of 998 characters for any line of text in an email message. If not properly encoded (e.g., with Base64 or Quoted Printable), a very long URL, especially when combined with other HTML content on a single line, could exceed this limit and cause delivery pain.
• URL Structure and Content: Spam filters (like SpamAssassin) might flag URLs that appear to contain a lot of random characters or hex strings, as this can be a characteristic of malicious or deceptive links, which sometimes coincide with longer URL structures. Find more about how multiple or long links affect email deliverability.

Key considerations

• Reputation is Key: The primary concern for deliverability related to URLs is not their length, but rather the reputation of the domain they link to. Linking to low-reputation or blacklisted domains is a major red flag for inbox providers. Learn more about how URL shorteners and domain reputation impact deliverability.
• Transparency of Redirects: Concealing the final destination (e.g., using stealth redirects or multiple redirect layers) can trigger spam filters, regardless of the initial URL length. Transparency builds trust.
• Proper Encoding: Ensure your sending platform properly encodes message bodies, especially when including long links, to prevent exceeding per-line character limits. This ensures compatibility across various MTAs and email clients.
• Monitor HTML Size: While HTML size itself might not directly affect deliverability below certain thresholds (e.g., 100KB), contributing factors like very long URLs can increase size, leading to clipping issues in email clients. Consider optimizing overall email code and limiting unnecessary elements to keep the size manageable.
• Avoid Deceptive Practices: Spammers often use deceptive URLs. Even if your URL is long, ensure it's clear and the linked content is consistent with expectations to avoid being mistaken for phishing attempts.

Key opinions

• Myth Persistence: Many marketers recall an older belief (from the 2000s) that long URLs negatively impacted deliverability, but personal experience suggests this is no longer a significant factor, as auditing for this issue did not change delivery results over the past decade.
• Clipping Concerns: The most tangible issue reported by marketers concerning long URLs is their contribution to increased HTML weight, which can lead to email clipping in clients like Gmail. Clipping can conceal essential elements, such as the unsubscribe footer, impacting user experience and compliance. This relates to how email code quality and size impact deliverability.
• Third-Party Tools: Some email testing services might still flag long URLs, but marketers speculate this could be a strategy to upsell link shortening services rather than an indicator of an actual deliverability problem.
• Outlook Limitations: Specific versions of Outlook (e.g., 2007, 2010, 2013) have documented limits for URL length. While long, these limits can still be problematic depending on how ESPs encode tracking links, underscoring the importance of testing across various clients.

Key considerations

• Prioritize User Experience: Even if not a direct deliverability blocker, excessively long URLs can look messy and unappealing to recipients. Using vanity URLs or ensuring concise display text for links improves readability and trust.
• Be Wary of Shorteners: While link shorteners might seem like a solution for long URLs, using generic public shorteners (e.g., Bitly) can introduce deliverability risks if those shorteners are associated with spam. It's important to understand if link shorteners are bad for email marketing.
• Focus on Link Quality: Marketers should prioritize the quality and reputation of the domains they link to. If linking to a problematic site, even a short URL could lead to issues. For broader context, see how email deliverability is impacted by various factors.
• Test Across Clients: Due to varying client-side rendering and legacy limits, marketers should always test their emails across different email clients to ensure links display correctly and do not cause clipping.

Key opinions

• Largely a Myth: Experts confirm that the idea of URL length being a major deliverability problem is largely a myth in current email environments, though individual organizational filters may have specific, arbitrary rules.
• Real URL Problems: The actual issues with URLs relate to attempts to conceal the final destination (e.g., stealth redirects), linking to low-reputation or scammy websites, and destinations serving different content to bots versus real users. These practices can significantly impact why emails go to spam.
• Historical Vulnerabilities: There's a vague historical memory among experts of early system vulnerabilities (like buffer overflows in specific MTAs) that might have been exploited by extremely long URLs, but such issues are rare today.
• SpamAssassin Rules: SpamAssassin specifically flags URLs that contain random hex letters or numbers, which often occur in conjunction with long URLs used in spam or malicious contexts. This indicates a focus on URL content and pattern rather than just length.
• Line Length Limits and Encoding: While not strictly a deliverability issue if handled correctly, a hard limit of 998 characters per line in an email message exists. If sending platforms fail to encode message bodies correctly (e.g., using Base64 or Quoted Printable), a long URL could push a line beyond this limit, causing delivery problems. This touches upon how unencoded URLs impact deliverability.

Key considerations

• MTA Responsibility: Ideally, the mail transfer agent (MTA) should handle line length limits through proper encoding, ensuring that even long URLs do not cause formatting or delivery issues.
• Content and Pattern Scrutiny: Instead of focusing on length, senders should be more concerned with whether their URLs contain patterns or characteristics that might be indicative of spam or malware, such as random characters or non-standard encoding. This contributes to your email domain reputation.
• ESP Encoding Practices: Senders should verify that their Email Service Provider (ESP) correctly encodes message bodies to prevent long URLs from creating problematic line lengths. A poorly configured ESP could inadvertently cause deliverability issues. Consider checking your emails using an email deliverability tester.
• Impact of Email Size: While HTML size generally has little impact below 100KB, extremely long URLs can increase email size, potentially slowing download times or contributing to clipping. Acoustic highlights this point, suggesting overall size optimization for better user experience, even if not a direct deliverability block.

Key findings

• Limit Number of URLs: Documentation (e.g., from Klaviyo) suggests that a large number of hyperlinked texts can be a red flag for inbox providers, as spammers frequently include many links. This implies quantity is more critical than individual URL length.
• Link Transparency: Documentation often warns against tactics that conceal the final destination of a link, such as certain URL shorteners or deceptive redirects, because they prevent ISPs from tracing the hosting domain. This is critical for security and trust, impacting ESP click tracking and deliverability.
• Spam Trigger: Placing entire, unmasked URLs directly into the body of a message can set off spam triggers because spammers often use this method with deceptive links.
• Email Size and Performance: While not directly about URL length, documentation notes that email file size can affect deliverability, as heavier emails take longer to download. Since long URLs contribute to email size, this is an indirect consideration. Acoustic cites Litmus, stating HTML under 100KB has no impact on deliverability.

Key considerations

• Link Quality and Reputation: Prioritize linking to reputable, trusted domains. The domain reputation associated with your links is a far more significant factor in deliverability than the length of the URL itself.
• Avoid Obfuscation: Documentation implicitly advises against techniques that hide the true destination of a link. This means avoiding overly complex or multi-layered redirects that obscure the final URL.
• Content Consistency: Ensure that the content at the destination URL is consistent with what is advertised in the email. Discrepancies can be flagged as deceptive practices, regardless of URL length. This can affect email deliverability rates.
• Optimize Email HTML: While individual URLs may be long, ensure the overall email HTML remains lean and well-structured to prevent clipping issues. This includes careful consideration of all elements that contribute to email size.