Do email spam filters scan commented-out sections of an email?

Yes, spam filters read the entire raw HTML code, including comments. While not visible to the recipient, the underlying code can still influence how the email is scored for deliverability. Filters prioritize efficiency and often scan all URLs without differentiating between visible and hidden ones.

How can a hidden broken link negatively affect sender reputation?

A broken link, even when commented out, can signal poor quality or a lack of attention to detail to spam filters. If the domain linked to has expired and been re-registered by a malicious entity, it could further damage your sender reputation and lead to your emails being blacklisted or sent to the spam folder.

Does this issue have a greater impact during domain warming?

During domain warming, your sender reputation is actively being built. Any negative signals, such as broken links, can hinder this process and make it harder to achieve good inbox placement. It's crucial to ensure all elements of your email are clean and optimized during this phase.

What is the best way to handle comments in email templates?

To prevent commented-out content from affecting deliverability, remove unnecessary code from your templates. If you need to comment for development, use your email service provider's or templating language's specific comment syntax, which often strips comments before sending.

Is the impact of a single hidden broken link significant?

While a single instance might not cause severe issues, the cumulative effect of multiple small code quality problems, including hidden broken links, can increase your email's spam score. This collective impact can be enough to trigger spam filters, especially for borderline emails.

Does a broken link in a commented out section of an email affect deliverability? - Sender reputation - Email deliverability - Knowledge base

The question of whether a broken link in a commented-out section of an email affects deliverability is a common one, and it highlights a crucial misunderstanding about how email systems work. Many believe that code within comments, especially HTML comments, is completely ignored by email clients and, by extension, by spam filters. This is often not the case.

While an HTML comment, such as , is designed to be invisible to the end user viewing the email, it remains part of the email's underlying code. This raw code is what mailbox providers and spam filters analyze to determine an email's legitimacy and intent. Therefore, what's hidden from your subscribers isn't necessarily hidden from the systems processing your email.

The consensus among deliverability experts is that even commented-out elements can, under certain circumstances, influence an email's path to the inbox. It's not a direct, guaranteed impact every time, but it introduces a potential risk that is best avoided for optimal email performance.

How email clients and spam filters process emails

When an email is sent, it's not just the visible content that goes through the recipient's mail servers. The entire raw HTML code, including comments, is transmitted and analyzed. Mailbox providers like

Google and

Yahoo employ sophisticated algorithms that scan the entire message source, not just what's displayed. This means that a broken link, even if commented out, can still be detected.

The rationale behind scanning all code, including hidden parts, stems from security and spam prevention. Malicious actors sometimes attempt to hide harmful content or phishing links within comments or obscure sections of an email's code, hoping to bypass basic filters. By thoroughly scanning all elements, mail providers aim to catch these hidden threats.

While a commented-out broken link might not cause an immediate delivery failure on its own, its presence contributes to the overall 'fingerprint' of your email. This fingerprint is a combination of many factors, including the quality of your HTML code, the integrity of your commented code, and other technical aspects. A high number of broken or suspicious links, even if hidden, can cumulatively raise your spam score. You can read more about how email links impact deliverability in this article.

The risk of broken links, even in comments

A broken link signals a lack of attention to detail, which can negatively affect your sender reputation. While end users may not see the link, the fact that it's broken can be interpreted by spam filters as a sign of a poorly maintained, potentially unprofessional, or even suspicious email. This is particularly true if the broken link points to a domain that has expired and been re-registered by a spammer or malicious entity. Such domains are often quickly added to email blocklists.

The impact of broken links is not limited to hidden elements. Even visible broken links can hurt your email open rates and overall campaign performance due to a poor user experience. When a broken link is present, whether visible or not, it can be flagged during automated email deliverability testing. This is a clear indicator that the element is being parsed and evaluated by pre-send checks.

During domain warming, where you are actively building your sender reputation, any potential negative signal can have an amplified effect. A broken link, even in a comment, can be one such signal that hinders your warming progress. Fixing these issues proactively contributes to a cleaner, more trusted sending profile.

The danger of expired domains

If a broken link in your email template points to an expired domain, that domain could be re-registered by spammers or bad actors. If this happens, your emails could inadvertently link to a malicious site, severely damaging your sender reputation and causing your emails to be blocked or sent to the spam folder. Always ensure all links, visible or hidden, point to valid and secure destinations.

The nuances of email filtering and link assessment

Understanding how email filters operate is key. They often prioritize efficiency. Rather than performing a computationally expensive HTML parse that determines if a link will actually render to the user, many filters use a simpler plaintext approach. This method extracts every URL present in the message's raw code, regardless of its placement within a comment or its visibility to the recipient. This approach is much lighter on resources and ensures no potentially harmful URL is missed.

Once all URLs are extracted, they are cross-referenced against various blacklists (or blocklists) and reputation databases. If any of these extracted URLs, even the commented-out ones, are associated with spam or malicious activity, it can negatively impact your email's deliverability. While some common, non-resolving prefixes or harmless broken links might be ignored over time, relying on this assumption is risky.

The cumulative effect of multiple small issues, such as broken links, excessive hidden code, or too many links in general, can be significant. Each issue might only add a tiny fraction to your spam score, but together they can push your email past the threshold, leading to it being diverted to the spam folder. This is why thorough testing and code hygiene are paramount.

Human perception

When you view an email, HTML comments are completely hidden. You only see the rendered content, which focuses on visual elements, text, and active links. To a human, a broken link in a comment is invisible and has no direct impact on their experience.

Filter perspective

Email spam filters and mailbox providers read the raw source code of an email. This includes all HTML, CSS, and comments. These systems are designed to detect hidden content, including URLs, for security and spam scoring purposes. A broken link, even in a comment, is detected and can influence deliverability.

Best practices for links and code in emails

The best approach to ensure optimal deliverability is to maintain clean and efficient email templates. This means removing any unnecessary code, including commented-out sections that are no longer relevant or contain problematic elements. Regularly audit your email templates to identify and fix any broken links, regardless of whether they are visible or hidden.

If you need to comment out code for developmental purposes, consider using server-side or templating language-specific comment syntax, if available. For example, many email service providers (ESPs) or templating engines offer their own commenting methods (e.g., Jinja2's {# #}) that strip the comments out before the email is sent, preventing them from ever reaching the recipient's mailbox provider. This ensures that the commented content never hits the wire, eliminating any potential deliverability risk.

Ultimately, every element in your email, from authentication protocols to content and code, plays a role in its deliverability. By proactively addressing even seemingly minor issues like broken links in commented-out sections, you contribute to a stronger sender reputation and better inbox placement. Think of it as spring cleaning for your email templates to ensure everything is optimized for success. You can also explore why your emails might be going to spam in 2024 to get a more holistic view.

HTML comments vs. Templating language commentshtml

<!-- This is an example of an HTML comment containing a broken link: -->
<!-- <a href="http://broken-link-example.com/page.html">Broken Link</a> -->

{# This is an example of a Jinja2 comment. This content will not be sent. #}
{# <a href="http://broken-link-example.com/page.html">Broken Link</a> #}

Views from the trenches

Best practices

Regularly audit your email templates to remove any unnecessary or commented-out code.

Prioritize fixing all broken links, even those not visible to recipients, to maintain trust.

Utilize native templating language comments to prevent content from hitting the wire.

Implement robust pre-send testing to catch and rectify any hidden deliverability issues.

Common pitfalls

Overlooking broken links within commented-out sections as harmless deliverability risks.

Assuming that invisible code is completely ignored by sophisticated email spam filters.

Neglecting to fix links to expired domains that could be re-registered by malicious actors.

Relying solely on visual checks instead of comprehensive code analysis for email integrity.

Expert tips

Understand that mailbox providers often scan all URLs for reputation, even in comments.

Real-world sending tests provide the most accurate assessment of deliverability impact.

Address any flagged issues during domain warming to build a strong sender reputation.

Consider that common external links might sometimes cause temporary filtering anomalies.

Marketer view

A marketer from Email Geeks observed a commented-out broken HTTP link consistently flagging on deliverability tests for a template, leading to internal debate about its real-world impact before domain warming.

2024-03-19 - Email Geeks

Expert view

An expert from Email Geeks emphasizes that real-world testing is essential to confirm the actual impact of such links, especially during domain or IP warming, as theoretical implications may not always materialize.

2024-03-19 - Email Geeks

The unseen impact on deliverability

While a broken link in a commented-out section of an email might not always lead to an immediate and catastrophic deliverability failure, it certainly introduces a preventable risk. Spam filters are designed to be thorough and will scan all elements of an email's code, visible or not. The presence of such a link can contribute to a higher spam score and negatively affect your sender reputation, especially during critical periods like domain warming.

To ensure the highest possible deliverability, it's always best practice to maintain clean, validated email templates free of any broken links or unnecessary code, even in commented-out sections. Proactive maintenance and thorough testing are your best allies in reaching the inbox consistently.