Summary
While HTML comments are primarily for developers to organize code, their presence in email campaigns can indirectly influence deliverability. The consensus among email marketing experts is that comments, especially if excessive, malformed, or containing suspicious content, can increase email file size and potentially trigger spam filters. Although not always a direct cause for flagging, comments can be misinterpreted as hidden text, a common spammer tactic, or simply contribute to overall email 'bloat,' affecting loading times and recipient engagement. This can also pose a risk if comments are visible in certain email clients or contain sensitive information.
Key findings
- Indirect Deliverability Impact: Commented code, while not always a direct spam trigger, can indirectly affect email deliverability primarily by increasing file size and potentially being misinterpreted by spam filters.
- Spam Filter Interpretation: Spam filters may view excessive or unusual comments as attempts to hide content, a tactic spammers use. This can lead to higher spam scores, especially if comments contain gibberish or suspicious keywords.
- Email Bloat: Comments add unnecessary 'bloat' to email HTML, increasing file size. Larger emails can load slowly, potentially affecting recipient engagement, and in some cases, may be truncated or delayed by ISPs.
- Visibility Risk: Certain email clients, particularly older or obscure ones, might inadvertently display HTML comments depending on formatting, making hidden text visible to recipients and potentially impacting professionalism.
- Unnecessary for Final Email: Experts largely agree that comments are unnecessary in the final, production version of an email, serving no functional purpose for the recipient or email client rendering.
Key considerations
- Remove Comments Pre-Send: The strongest recommendation is to remove all HTML comments from emails before sending them. This ensures the cleanest possible code and minimizes potential deliverability issues.
- Minimize & Validate Comments: If comments absolutely must be included, keep them minimal, concise, and ensure they are valid HTML comments. Avoid using them to hide content or break up keywords.
- Clean Code Practices: Prioritize clean, lean HTML code by removing any unnecessary elements, including comments, excess whitespace, and unused CSS, to optimize email performance and deliverability.
- Content Sensitivity: Be mindful of any content within comments, especially for clients in highly regulated industries like finance, as even hidden text could be scrutinized for liability or compliance reasons.
- Thorough Testing: Always test your email campaigns across various email clients and spam filter services to identify any unexpected rendering issues or spam flagging related to your code, including comments.
What email marketers say
10 marketer opinions
While essential for developers, commented code in HTML emails generally offers no benefit to the end-user and can negatively impact deliverability. Email marketing experts advise that the presence of comments, especially if extensive or containing unusual content, can increase email file size and potentially trigger spam filters. These filters may misinterpret comments as attempts to hide text, a common spammer tactic. Although some filters might ignore valid, minimal comments, the consensus leans towards removing them entirely to ensure the cleanest possible code, avoid accidental display in certain email clients, and prevent any unintended negative effects on an email's spam score.
Key opinions
- Spam Filter Scrutiny: Spam filters, particularly more sophisticated ones, can analyze comments. They may flag emails if comments are excessive, malformed, or contain gibberish, as these are tactics spammers use to bypass content filters.
- Increased Email Size: Comments contribute to the overall file size of an email. While not a direct spam trigger, large email sizes can slow loading times, potentially affect recipient engagement, and may be viewed suspiciously by some older spam filters.
- Hidden Text Interpretation: Even well-intended comments can be misinterpreted by spam filters as an attempt to hide text, a common spamming technique. This misinterpretation can lead to an increased spam score.
- Client-Specific Rendering: Some email clients, particularly Microsoft's Hotmail and certain versions of Outlook, have been noted for sometimes displaying or misinterpreting text within style tags or comments, potentially exposing what should be hidden code.
- Developer vs. Sender Need: Comments serve a purpose for developers to organize and explain code. However, they provide no functional benefit for the end-recipient or the final rendered email, making them unnecessary for the sent version.
Key considerations
- Pre-Send Comment Removal: It is strongly advised to remove all HTML comments from email code before sending. This practice ensures optimal deliverability by eliminating potential issues related to file size and spam filter interpretation.
- Code Cleanliness: Adopt a rigorous approach to code hygiene, including the removal of all unnecessary elements like comments, excessive whitespace, and unused CSS, to improve email loading speed and reduce the chance of spam flags.
- Testing Across Clients: Always conduct thorough testing of your emails across various email clients and through spam filter services. This helps identify any unintended rendering issues or spam flagging that might arise from commented code or other hidden content.
- Avoid Hidden Content: Never use comments or any other method to intentionally hide text or content within your email. Spam filters are highly sensitive to such tactics, often associated with malicious intent.
- Minimizing Comments If Used: If, for specific development reasons, comments absolutely cannot be removed, ensure they are valid HTML comments, minimal in quantity and size, and contain no suspicious or gibberish text that could be misinterpreted by filters.
Marketer view
Marketer from Email Geeks explains that spammers use gibberish in non-visible comments to bypass content filters, suggesting that large amounts of comment code, especially gibberish, can make spam filters punish the email. She recommends testing and ultimately using a script to generate a clean version of the email by removing all comments, while allowing developers to retain original commented code.
26 Jan 2025 - Email Geeks
Marketer view
Marketer from Email Geeks explains that good Bayes spam filters typically remove HTML from valued tags, so comments might not count against an email, especially if they are three characters or less. However, he agrees it's bloat and recommends keeping quantity and size of comments to a minimum. He also notes that Microsoft's Hotmail often shows spam filled with junk text hidden in style tags, which implies similar issues.
10 Dec 2024 - Email Geeks
What the experts say
4 expert opinions
Email marketing experts generally agree that commented code in emails, while seemingly harmless, can negatively impact deliverability. Their presence adds unnecessary file size, contributing to 'bloat,' and can cause spam filters to scrutinize the email more closely. Filters may misinterpret comments as attempts to hide malicious content or to bypass keyword detection, a common spammer technique. Although their impact is often secondary to other factors, comments containing suspicious keywords, or those used excessively, increase the risk of an email being flagged. For highly regulated sectors like finance, the potential for liability means even hidden text in comments warrants removal.
Key opinions
- Bloat & File Size Increase: Commented code adds unnecessary bytes to an email's file size, contributing to 'bloat' that can slow loading times and be flagged by some spam filters.
- Spam Filter Misinterpretation: Spam filters may scrutinize comments, misinterpreting them as attempts to hide malicious content or evade keyword detection, a common tactic used by spammers.
- Keyword Trigger Risk: Even benign comments can inadvertently trigger spam filters if they contain suspicious keywords or strings, regardless of their hidden nature.
- Indirect Deliverability Impact: While not always a direct cause of a spam flag, comments can indirectly contribute to an email's overall spam score, especially when excessive or containing questionable content.
- Financial Sector Liability: Experts advise that financial institutions, due to higher liability thresholds, must be particularly diligent in removing comments that could contain any impactful or sensitive information.
Key considerations
- Remove Comments Pre-Send: The strongest recommendation from email deliverability experts is to remove all HTML comments from email code before sending to ensure optimal deliverability and minimize potential issues.
- Concise & Legitimate Use: If comments are absolutely necessary, such as for MSO conditional statements, keep them minimal, concise, and ensure they serve a clear, legitimate purpose without containing suspicious content.
- Avoid Suspicious Keywords: Never include suspicious keywords or attempt to break up words within comments, as this is a known spammer tactic that can trigger spam filters.
- Industry-Specific Scrutiny: For highly regulated industries like finance, the potential for increased liability means that any text, even within comments, should be carefully considered and ideally removed to avoid unintended scrutiny.
- Thorough Deliverability Testing: Always test your email campaigns across diverse email clients and using spam filter services to identify any unexpected rendering issues or deliverability flags caused by commented code.
Expert view
Expert from Email Geeks always recommends removing comment code from emails because it is unnecessary bloat and can trip up spam filters.
12 Oct 2023 - Email Geeks
Expert view
Expert from Email Geeks suggests that financial clients may have a higher threshold for liability, making it important to remove comments that could say anything impactful.
8 Jun 2024 - Email Geeks
What the documentation says
4 technical articles
Major email service providers and deliverability experts advise removing commented code from emails before sending. While HTML comments do not directly trigger spam filters, they undeniably increase the email's file size. This 'bloat' can lead to emails loading slowly, potentially being truncated by internet service providers, or, when combined with other factors, may contribute to a slightly higher spam score. The overarching recommendation is to maintain clean, lean, and concise email code by stripping out all unnecessary elements, including comments, to ensure optimal deliverability and user experience.
Key findings
- Increased File Size: Commented code, along with other unused elements, increases the overall file size of an email campaign.
- Indirect Deliverability Influence: While not a direct cause for flagging, increased email size from comments can indirectly affect deliverability.
- Potential for Truncation: Larger email messages may be truncated by certain Internet Service Providers (ISPs), affecting how recipients view the content.
- Slight Spam Score Factor: Excessive hidden elements, even comments, can subtly contribute to an email's spam score, especially when combined with other suspicious factors.
- Clean Code Priority: It is a widely accepted best practice to keep email HTML lean and concise by removing all unnecessary components like comments.
Key considerations
- Proactive Code Minification: Consistently remove all unnecessary characters, including comments, excess whitespace, and unused CSS, to reduce email file size.
- Enhance User Experience: Leaner emails load faster, which improves the recipient's experience and reduces the chance of truncation by ISPs.
- Mitigate Minor Spam Risks: Eliminating comments helps prevent any minor contributions to a higher spam score, particularly when combined with other elements.
- Embrace Clean Coding Habits: Adopt a general best practice of maintaining concise and efficient email code, ensuring every element serves a functional purpose.
- Consider Overall Message Size: Be mindful that large message sizes, partly due to comments, can lead to delivery delays or content truncation.
Technical article
Documentation from Mailchimp explains that while HTML comments do not directly affect rendering, it is a general best practice to keep email code clean and concise by removing unnecessary white space, comments, and unused CSS, as they add to the file size.
22 Jun 2021 - Mailchimp
Technical article
Documentation from Twilio SendGrid advises minifying HTML by removing unnecessary characters, including comments. While comments do not directly harm deliverability, they increase email size, which can be a minor factor in deliverability as large messages might be truncated or delayed by some ISPs.
26 Feb 2025 - Twilio SendGrid
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