How do HTML email size, TLD, attachments, and domain reputation affect deliverability and open rates?
Michael Ko
Co-founder & CEO, Suped
Published 31 May 2025
Updated 16 Aug 2025
7 min read
Achieving excellent email deliverability and strong open rates is a complex endeavor, influenced by numerous technical and content-related factors. It is not just about the message itself, but also how that message is constructed and perceived by recipient mail servers.
Among these factors, the HTML email size and complexity, your chosen top-level domain (TLD), the presence of attachments, and your domain reputation all play critical roles. Each element can subtly or significantly impact whether your email lands in the inbox or gets diverted to the spam folder.
Understanding how these technical details interoperate is essential for optimizing your email campaigns and ensuring your messages reach their intended audience. We will explore each of these factors and provide best practices to help you improve your email performance.
HTML email size and complexity
The size of your HTML email can directly affect its deliverability. Larger email files, especially those exceeding certain thresholds, can trigger spam filters or cause issues with image loading for recipients. Mailbox providers like Google and Yahoo Mail have internal limits, often around 100KB, beyond which emails may be truncated or experience delayed rendering.
This truncation can lead to a poor user experience, as subscribers might not see your full message or all critical calls to action, potentially impacting your open rates and engagement metrics. Overly complex HTML structures, while not directly increasing file size in all cases, can also contribute to rendering issues across different email clients.
Optimal email size guidelines
To ensure broad compatibility and reduce deliverability risks, aim to keep your total email size, including images and other embedded elements, under the recommended threshold. This applies to the raw HTML code as well as the entire email package.
Minimize code: Remove unnecessary spacing, comments, and redundant HTML elements.
Optimize images: Compress images and host them externally rather than embedding them.
Conditional content: If using conditional content for different languages or regions, ensure your Email Service Provider (ESP) processes these conditions server-side. This ensures only the relevant block is sent, not the entire, larger HTML file.
Even with conditional logic, if the full HTML code including all conditional blocks is excessively large before rendering by the ESP, it could still present issues. It is important to remember that image loads can be affected if the overall size pushes an email over a specific threshold, especially for providers like Google. Maintaining a lean HTML structure is a good practice for optimal deliverability and user experience. You can learn more about how email code quality affects deliverability.
The role of TLDs in deliverability
Your top-level domain (TLD) is the suffix at the end of your domain name, such as .com, .org, or .de. While major, reputable TLDs like .com or country-code TLDs (ccTLDs) generally do not negatively impact email deliverability, the choice can influence recipient perception and, in some cases, filtering behavior. If recipients are accustomed to receiving emails from a specific ccTLD, an unexpected switch to a generic TLD like .com could subtly impact their trust or filter behavior.
For instance, if your French customers are used to emails from a .fr domain, receiving an email from a .com domain might feel slightly off, even if it is legitimate. This can affect how they interact with your emails, potentially leading to lower engagement or even marking your emails as spam, which then harms your domain reputation. Generally speaking, a reputable TLD has negligible effect, but recipient expectations can matter.
Country-code TLDs (e.g. .fr, .de)
Local relevance: Can enhance trust and familiarity for country-specific audiences.
Expected behavior: Recipients may expect emails from a local domain.
Potential for siloed reputation: Reputation built on one ccTLD may not fully transfer.
Generic TLDs (e.g. .com, .org)
Widespread acceptance: Generally recognized as legitimate worldwide.
Brand consistency: Can provide a consistent global brand identity.
Established reputation: Well-established domains may have built trust over time.
While the technical impact of a standard TLD on deliverability is often minimal (as explained by Bobby Jimenez), a sudden change can still raise flags for recipients who are used to your usual sending patterns. Mailbox providers (ISPs) often consider sender history and consistency. Learn more about how the TLD affects email deliverability and cold email deliverability specifically.
Attachments and their deliverability implications
Directly attaching documents to your emails can significantly impact deliverability and open rates. Attached files increase the overall size of the email, which, as discussed, can lead to truncation or filtering by mailbox providers. More importantly, attachments, especially common formats like PDFs, are often viewed with suspicion by spam and malware filters because they are frequently used for phishing and distributing malicious software.
Risks of email attachments
While a shipping label is a legitimate use case for an attachment, there are inherent risks. Consider linking to documents hosted securely online rather than attaching them directly to minimize deliverability risks and enhance tracking capabilities.
Increased email size: Pushes emails over size limits, leading to truncation or rejection.
Spam filter triggers: Certain file types (e.g., PDFs, executables) are flagged by anti-spam and malware filters.
Poor tracking: Direct attachments offer no insight into whether the recipient viewed the document.
Instead of attaching, consider providing a secure link to the document. This approach offers several benefits: it keeps your email size down, avoids potential spam triggers, and allows you to track clicks, giving you a more robust metric for engagement than just email opens. While a shipping label might be a valid exception for direct attachment, exploring alternatives is crucial for overall deliverability. You can find more information on how PDF attachments affect deliverability.
Domain reputation and sender trust
Your domain reputation is arguably the most critical factor influencing email deliverability. It is a score that mailbox providers assign to your sending domain, based on various historical and ongoing sending patterns. A strong domain reputation indicates trustworthiness, while a poor one can lead to your emails being directed to the spam folder, rejected entirely, or even result in your domain being placed on a blacklist (or blocklist).
Factors such as spam complaint rates, bounce rates, subscriber engagement (opens, clicks), and whether your domain has been involved in sending spam all contribute to this score. Maintaining a consistent sending volume and content quality is vital. A sudden change in sending patterns, such as an increase in volume or a change in the primary TLD, can temporarily impact your reputation if not managed carefully.
Factor
Impact on domain reputation
Spam complaints
High rates severely degrade reputation, leading to blocklists (blacklists) and spam folder delivery.
Bounce rates
High hard bounce rates indicate a poor list hygiene and negatively impact trust. Soft bounces are less critical but still monitored.
Spammy content or questionable links can hurt reputation, regardless of other factors. Mailgun also notes content reputation as a major factor.
A robust domain reputation is built over time through consistent, positive sending practices. It is crucial to monitor your domain’s health using tools like Google Postmaster Tools to identify and address any issues promptly. A drop in open rates, particularly for specific providers like Outlook or GMX, can be an early indicator of a degrading domain reputation. You can discover more about how sending practices impact domain reputation.
Views from the trenches
Best practices
Always link to external documents rather than attaching them to reduce email size and avoid spam triggers, especially for PDFs.
Segment your audience by country and send from country-specific TLDs (e.g., .fr for France, .de for Germany) to align with recipient expectations and avoid confusion.
Keep your email HTML lean by minimizing unnecessary code, comments, and inline CSS to reduce overall email size and improve loading speed.
Regularly monitor your domain and IP reputation using tools like Google Postmaster Tools to proactively identify and address deliverability issues.
Ensure your Email Service Provider (ESP) processes conditional content server-side, so only the necessary HTML block is sent to each recipient.
Common pitfalls
Sending emails with large attachments can significantly increase email size, leading to slower load times or emails being clipped by mailbox providers like Gmail.
Using a generic TLD like .com when recipients are accustomed to country-specific domains can sometimes lower perceived relevance and engagement.
Overly complex HTML structures, even with conditional logic, can increase the risk of rendering issues or being flagged by spam filters if not properly optimized.
Ignoring subtle drops in open rates on specific domains (e.g., Hotmail/Outlook, GMX) can be an early indicator of a degrading sender reputation or bulk folder placement.
Failing to warm up a new sending domain or TLD can lead to immediate deliverability issues as mailbox providers will have no historical reputation data.
Expert tips
Expert from Email Geeks says: If your conditional content is processed by your ESP, the size of the email will only be that of the single, relevant block, not the sum of all conditional blocks.
Expert from Email Geeks says: A legitimate TLD like .com or .de should have a negligible impact on deliverability and open rates.
Expert from Email Geeks says: Attachments can increase email size, potentially interfering with image loads at providers like Google if the size exceeds a certain threshold.
Expert from Email Geeks says: PDFs in attachments are often viewed with suspicion by spam and malware filters, which can negatively impact deliverability.
Expert from Email Geeks says: Linking to documents provides a more robust metric of recipient interaction compared to just tracking email opens.
Expert view
Expert from Email Geeks says that if an attached document is truly attached rather than linked, it could affect email delivery, though a shipping label is a reasonable exception.
2021-12-07 - Email Geeks
Expert view
Expert from Email Geeks says that email attachments increase mail size, which can interfere with image loading at Google if it exceeds a certain size threshold.
2021-12-07 - Email Geeks
Optimizing for email success
Email deliverability and open rates are influenced by a delicate balance of technical considerations and sender reputation. The size and complexity of your HTML email, the choice of your TLD, the inclusion of attachments, and your overall domain reputation all contribute to whether your message successfully lands in the inbox. Ignoring these factors can lead to reduced visibility and missed opportunities for engagement.
By optimizing your HTML structure, making informed decisions about TLDs, carefully considering attachment alternatives, and diligently managing your domain’s sending reputation, you can significantly improve your email program’s performance. Remember that consistent monitoring and adaptation to mailbox provider requirements are key to long-term success.
Ultimately, a holistic approach that considers all these elements will yield the best results for your email deliverability and open rates. Prioritizing technical health alongside compelling content ensures your messages not only reach their audience but also resonate effectively.
Google. Maintaining a lean HTML structure is a good practice for optimal deliverability and user experience. You can learn more about how email code quality affects deliverability.
Outlook or GMX, can be an early indicator of a degrading domain reputation. You can discover more about how sending practices impact domain reputation.
