Does commented code in emails affect spam filters and deliverability?
Matthew Whittaker
Co-founder & CTO, Suped
Published 7 May 2025
Updated 19 Aug 2025
7 min read
When building email templates, especially for complex campaigns, it is common practice for developers to use HTML comments. These comments are invaluable for internal documentation, marking sections for dynamic content insertion, or temporarily disabling parts of the code during development. However, a question often arises: do these seemingly harmless comments affect email deliverability or trigger spam filters? It's a valid concern, as anything that could hinder an email from reaching the inbox is worth investigating.
The short answer is that while well-placed, concise comments generally pose little risk, certain types or excessive use of commented code can indeed impact how spam filters perceive your email. Understanding why and how this happens is crucial for maintaining good inbox placement and ensuring your messages are delivered effectively.
How spam filters interact with commented code
Email spam filters are sophisticated systems designed to protect recipients from unwanted or malicious content. They analyze various aspects of an email, including sender reputation, authentication records, and, importantly, the content and structure of the email itself. When it comes to HTML code, these filters look for patterns and anomalies that might indicate spam-like behavior.
While comments are not visible to the end-user, they are part of the email's overall HTML structure. Some spam filters may scan the entire raw HTML source, including comments, as part of their heuristic analysis. If these comments contain suspicious keywords, obfuscated content, or simply add excessive weight to the email, they can contribute to a higher spam score. This is particularly true for less advanced or older spam filtering systems.
Spammers have historically exploited HTML comments by stuffing them with keywords or irrelevant text to try and bypass content filters. This practice, known as keyword stuffing or hidden text, can lead legitimate emails that use comments to be inadvertently flagged. Even if the comments are benign, their mere presence might trigger a rule designed to catch such deceptive practices. Therefore, being mindful of what you place in comments is important for email deliverability.
Bloat and content within comments
One of the primary concerns with commented code is the potential for email bloat. Every character in your email's HTML contributes to its overall file size. While modern email clients and servers can handle larger email sizes, excessively large emails can raise red flags for spam filters. If your email is disproportionately large due to extensive comments or commented-out sections of code, it might be viewed as suspicious.
For instance, an email over 100KB in HTML size is sometimes considered large, and if much of that size comes from non-essential commented code, it can negatively affect deliverability. This is because spam filters may interpret large, unwieldy code as an attempt to conceal malicious content or simply deem it inefficient and unprofessional, which can lower your email's spam score.
Beyond size, the specific content within comments matters. Comments containing common spam trigger words, unusual character sets, or large blocks of seemingly random text can be problematic. Even if intended for development purposes, such content can mimic patterns seen in phishing attempts or other undesirable emails. This is particularly relevant for businesses in sensitive sectors, like financial services, where email content is scrutinized more heavily to prevent fraud.
Best practices for comments
While completely avoiding comments might not always be practical or necessary, adopting best practices can mitigate risks. Consider using concise comments for critical markers rather than verbose explanations. For example, <!-- HEADER --> is preferable to a lengthy instruction block. Additionally, tools are available to help remove HTML comments from emails before sending, which can significantly improve deliverability by reducing code bloat and removing potential red flags.
Clean and efficient code is always a good practice in email development. This includes minimizing unnecessary HTML, inline styles, and yes, even comments. A streamlined code base not only reduces file size but also presents a cleaner structure to spam filters, which can positively influence your overall email deliverability.
Broader impact on deliverability
While commented code can play a role, it is rarely the sole reason an email lands in the spam folder. Far more influential factors include your sender reputation, email authentication (SPF, DKIM, DMARC), recipient engagement, and the overall quality of your email content (e.g., avoiding spam trigger words). Even a perfect HTML email with no comments can fail to deliver if other elements are not optimized.
A strong sender reputation, built on consistent good sending practices, is your best defense against spam filters. If your domain and IP address have a history of sending legitimate, engaged emails, minor issues like a few comments are less likely to significantly impact your deliverability. Conversely, a poor reputation means even small coding quirks can amplify negative signals to filters.
It is always recommended to implement comprehensive strategies to avoid spam filters, focusing on the most impactful aspects first. This includes ensuring proper email authentication, maintaining clean email lists, monitoring engagement, and crafting relevant content. Once these foundational elements are solid, then you can refine code-related aspects like comments and file size.
Code optimization
Problem: Excessive commented code or large commented-out sections of HTML can increase email file size, potentially leading to deliverability issues and higher spam scores. It can also be perceived as a tactic used by spammers to hide content.
Content transparency
Problem: Comments containing suspicious keywords, irrelevant text, or obfuscated characters may be flagged by spam filters looking for deceptive content. This is a common tactic to bypass filters.
Solution: minification
Solution: Use a build process to strip out comments and unnecessary whitespace before sending. This creates a clean, optimized HTML file specifically for deployment, while developers retain commented versions for their work. This is part of maintaining email code quality.
Solution: concise comments
Solution: If comments are essential for workflow, keep them minimal and strictly informative. Avoid using them to hide text or for extensive, unnecessary explanations. For more on this, review best practices for comments.
Testing and automation
To determine the actual impact of commented code on your specific email campaigns, conducting tests is invaluable. You can use an email deliverability testing tool to send versions of your email, one with comments and one without, to various inbox providers and check the spam folder placement. This direct observation will provide the most accurate assessment for your unique email content and sending setup.
If testing reveals that comments are indeed causing issues, automating their removal is an efficient solution. Many email service providers (ESPs) offer options to minify HTML or remove comments as part of their sending process. Alternatively, you can implement a pre-processing step in your development workflow to strip comments before deployment, ensuring that your production emails are as lean and clean as possible.
Example of commented HTML email codehtml
<!DOCTYPE html>
<html>
<head>
<title>Email Title</title>
<!-- Basic styles for email clients -->
<style>
body { font-family: Arial, sans-serif; }
/* Remove this comment block for production */
.button { background-color: #007bff; color: white; padding: 10px 20px; }
</style>
</head>
<body>
<!-- Start of header section -->
<div class="header">
<h1>Welcome!</h1>
</div>
<!-- End of header section -->
<!-- Body content starts here -->
<p>This is the main content of your email.</p>
<!-- [DYNAMIC_CONTENT_AREA] -->
<p>More details here.</p>
<!-- Footer with legal text -->
<div class="footer">
<p>Copyright 2024</p>
<!-- Legal disclaimer placeholder -->
</div>
</body>
</html>
Ultimately, the decision to remove or retain commented code should be balanced with its utility for your team and its measured impact on deliverability. For crucial campaigns where every percentage point of inbox placement matters, being proactive about code cleanliness, including comments, is a worthwhile investment. This approach contributes to a more robust and reliable email program.
Views from the trenches
Best practices
Maintain clean email lists, regularly removing inactive or invalid addresses to improve engagement and sender reputation.
Always include a plain text version of your email alongside the HTML version for better compatibility and spam filter bypass.
Use email authentication methods such as SPF, DKIM, and DMARC to verify your sending domain and prevent spoofing.
Monitor your sender reputation and DMARC reports regularly to catch and address potential deliverability issues early.
Design email templates with minimal HTML, avoiding excessive inline styles or unnecessary code elements.
Common pitfalls
Over-relying on image-only emails, which can increase spam scores because filters cannot read text within images.
Including excessive or lengthy HTML comments, which can bloat email size and trigger spam filters.
Using a high density of spam trigger words in subject lines or body copy, leading to immediate spam flagging.
Not having a clear unsubscribe option, which can lead to higher spam complaints and negative sender reputation.
Sending emails to unengaged recipients, signaling to ISPs that your content is not desired.
Expert tips
Even minimal, concise HTML comments should be considered for removal in production emails if you're facing stringent spam audits.
Content hidden by low color contrast, such as white text on a white background, is a classic spammer trick that filters actively look for.
Financial content in emails is under extreme scrutiny; avoid common financial spam words and phrases where possible.
While specific content elements like comments or dollar signs can contribute, overall sender reputation and recipient engagement are usually the dominant factors in deliverability.
Implement a build script to automatically strip comments and unnecessary bloat from your email HTML before deployment.
Marketer view
Marketer from Email Geeks says putting tons of gibberish in non-visible comments is a known spammer tactic to get around content filters, so it depends on the content of the comments.
2018-08-14 - Email Geeks
Marketer view
Marketer from Email Geeks says that extra bloat from comment code is unnecessary and can trip up spam filters, so it is always recommended to remove it from emails.
2018-08-14 - Email Geeks
Our recommendations
While HTML comments in emails generally don't directly cause a significant deliverability issue on their own, their potential for bloat and misuse by spammers means they can, under certain circumstances, contribute to a higher spam score. Modern spam filters are sophisticated and consider many factors beyond just code comments. However, clean and optimized code is always beneficial.
The key takeaway is to prioritize brevity and relevance in your comments. If comments are essential for your workflow, ensure they are minimal and do not add excessive weight or suspicious content. For maximum deliverability, consider stripping them out entirely before sending. Ultimately, focusing on strong sender reputation, proper authentication, and engaging content will have a far greater impact on your inbox placement than the presence of well-managed commented code.
