Summary
The effect of broken links in HTML comments on email deliverability is complex and multifaceted. While browsers ignore comments, crawlers, including those used by search engines and potentially email filters, may still process the URLs found within them. This processing can impact a sender's reputation, especially if numerous broken links are present, signaling poor website or email maintenance. Furthermore, because user experience is a key ranking factor, the presence of broken links, regardless of whether they are in comments, can degrade this and have an indirect negative impact on deliverability and SEO. Therefore, it's crucial to maintain valid URL structures, implement schema markup correctly, and regularly check for and rectify broken links to ensure optimal performance and a positive user experience. Finally, it is key to test these within your own sending environment as filtering is complex and what others experience might be different to you.
Key findings
- Crawler Visibility: Crawlers, used by search engines and potentially email filters, can access and process URLs within HTML comments, despite browsers ignoring them.
- Reputation Impact: Numerous broken links, even in comments, can negatively impact a sender's reputation, indicating poor maintenance.
- User Experience Degradation: Broken links, regardless of their location, negatively affect user experience, harming deliverability and SEO.
- Plaintext URL Extraction: Some email filters use plaintext tools that extract all URLs.
- Real-World Impact: Impact on deliverability during the warm-up phase.
Key considerations
- Maintenance: Regularly check for and fix broken links, even in HTML comments.
- Proper URL Structure: Ensure valid URL structures to prevent broken links in the first place.
- Accurate Schema: Implement schema markup correctly to avoid misinterpretations by crawlers.
- Testing: Test within your own sending environment.
- Filter Logic: Be aware that email filters may evaluate all links in an email, regardless of whether they are commented out.
What email marketers say
10 marketer opinions
The impact of a broken link in a commented-out section of an email on deliverability is complex and not definitively negative, but there are potential risks. While HTML comments are generally ignored by browsers, search engine crawlers and some email filters might still process them. This means a broken link, even in a comment, could contribute to a negative perception of the sender's quality, especially if there are many broken links. The user experience is a ranking factor in SEO and deliverability, and broken links can hurt the user experience. Also filtering writers could use plaintext tools to extract every URL from the message. Website maintenance is important for SEO and deliverability, and broken links should be fixed.
Key opinions
- Crawler Visibility: Search engine crawlers and some email filters *can* see and process content within HTML comments.
- Reputation Impact: Broken links contribute to a negative perception of site/sender quality, especially if there are many.
- User Experience: Broken links can negatively affect user experience, which is a ranking factor for both SEO and email deliverability.
- Crawling Efficiency: Broken links might waste crawl budget, signaling a less-than-ideal site structure.
- Plaintext URL Extraction: Some email filters use plaintext tools to extract all URLs.
Key considerations
- Filter Logic: Email filters may evaluate all links in an email.
- Comment Parsing: Consider that it's worth implementing any schema or URLs in the proper location in the HTML, or it may be misinterpreted.
- Quantity Matters: A single broken link is unlikely to cause significant harm, but a large number of them could.
- Site Maintenance: Regularly check for and fix broken links to maintain a healthy website and positive reputation.
- Template Quality: Ensure email templates don't contain unnecessary or outdated links, even in comments.
Marketer view
Email marketer from Email Geeks shares that it's best to picture yourself as the writer of the filter. Filter writers need to evaluate all the links in an email to determine if any of them have a bad associated reputation. They will likely use a plaintext tool that extracts every URL from the message, regardless of where it appears in the message or whether it would actually render to the user, something much lighter from a resource perspective. What they do with those URLs is a different matter, like Laura said some URLs will be common to most email messages and will likely get flagged to be ignored because they are too common to be used for reputation purposes, but they will no doubt grab all the URLs.
15 Aug 2021 - Email Geeks
Marketer view
Email marketer from Shopify explains that a good user experience is good for SEO. They suggest that any broken link will impact user experience, so fix them.
8 Apr 2022 - Shopify
What the experts say
4 expert opinions
Experts agree that broken links in commented-out sections of emails can potentially affect deliverability, although the real-world impact is variable. Reputation and trust are built over time, and broken links can undermine this if they appear malicious. If the link or hostname correlates with unwanted mail, it's detrimental. Mailbox providers may key on these URLs, sharing your reputation with other users of the template. Testing in a real-world environment is crucial to determine the actual impact.
Key opinions
- Reputation Impact: Broken links can undermine reputation and trust, especially if they appear malicious.
- Correlation with Unwanted Mail: Links or hostnames correlated with unwanted mail negatively impact deliverability.
- Shared Reputation: Mailbox providers may key on commented-out URLs, sharing your reputation with other template users.
- Real-World Testing: The true impact of broken links is best determined through testing in a real-world environment.
Key considerations
- Template Comments: Use templating language comments instead of HTML comments to avoid sending the content over the wire.
- Impact Variability: The impact may vary; filtering is complex, and deliverability may or may not be affected.
- Warm-Up Phase: Broken links are more likely to impact deliverability during domain/IP warm-up.
- Building Trust: Deliverability is about building reputation and trust over time, and links are one component of that.
Expert view
Expert from Word to the Wise explains that common deliverability issues arise from list quality, authentication and reputation. A broken URL can cause issues as it will not be seen as a good link to be pointing to.
11 Jul 2024 - Word to the Wise
Expert view
Expert from Email Geeks explains that most people and tools will point out broken links can harm delivery, but the real proof is in underlying delivery. The only way to tell if it's going to make a difference in a real world environment is to send mail in a real world environment. It might affect deliverability, especially during warmup, but it also might not. Filtering is complex. Very occasionally she has seen reputation engines go a bit bonkers and block mail with <http://fonts.googleapis.com|fonts.googleapis.com>. It affects a lot of wanted mail, so it generally resolves pretty quickly.
25 May 2022 - Email Geeks
What the documentation says
4 technical articles
Technical documentation suggests that while HTML comments are not rendered by browsers, crawlers can still see the content within them. Broken links, in general, are bad for user experience and crawlability, regardless of their location. Adhering to URL standards and implementing schema markup correctly is important, as misinterpretations can lead to errors. Even though comments aren't directly affecting rendering, the impact on deliverability comes from how crawlers process this information.
Key findings
- Crawler Access: Crawlers can still see content within HTML comments, despite not being rendered by browsers.
- URL Standards: Adhering to URL standards prevents broken links in the first place, ensuring proper formatting.
- Schema Markup: Incorrectly implemented schema markup within comments can lead to errors and indexing issues.
- Broken Links: Broken links are detrimental to user experience and can hinder search engine crawling.
Key considerations
- Regular Checks: Regularly check for and fix broken links to ensure a positive user experience.
- HTML Comments: Ensure proper implementation of markup within HTML comments to avoid misinterpretations.
- Impact on Crawlers: Consider how crawlers might process content within HTML comments.
- Content Interpretation: Remember that the way crawlers see and interpret the content will likely influence how deliverability is affected.
Technical article
Documentation from W3C explains that HTML comments `<!-- ... -->` are not rendered by the browser. While not directly addressing deliverability, this suggests that commented-out code *shouldn't* directly affect how mail servers interpret the active content. But if a crawler is looking at the raw HTML then it will see the links. This can be different to rendering
11 Dec 2023 - W3C
Technical article
Documentation from IETF explains the proper formatting and structure of URLs. While not directly addressing commented-out links, adhering to URL standards can help prevent broken links in the first place. Ensuring valid URL syntax is crucial for proper rendering and crawlability.
25 Apr 2025 - IETF
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