How does email code quality and size impact email deliverability?
Michael Ko
Co-founder & CEO, Suped
Published 13 Jun 2025
Updated 15 Aug 2025
6 min read
Email deliverability is a complex puzzle with many pieces. While we often focus on sender reputation, list hygiene, and authentication protocols, the underlying code quality and overall size of your email can significantly influence whether your messages land in the inbox or the spam folder.
It is easy to overlook the technical aspects of email creation, especially when using drag-and-drop editors or pre-built templates. These tools, while convenient, can sometimes generate bloated or less-than-optimal HTML that, unbeknownst to us, might be raising red flags with mailbox providers.
Understanding how various elements like HTML file size, CSS structure, and image optimization affect deliverability is crucial. We will explore these factors and offer practical strategies to ensure your emails are not only visually appealing but also technically sound for optimal inbox placement.
The impact of email size
One of the most immediate impacts of your email's construction relates to its overall size. This includes the HTML code itself, embedded images, and any attachments. Mailbox providers, such as Gmail, have specific size thresholds, and exceeding these can lead to messages being truncated (clipped) or even rejected.
A common threshold is around 102KB for the HTML body. If your email's code, including inline CSS, goes beyond this, recipients might not see the entire message, requiring them to click a link to view the full content in a browser. This can hurt engagement and signal to mailbox providers that your content might be too heavy, potentially impacting your overall sender reputation.
Beyond HTML, the size of email images and other media elements contributes significantly to the total email weight. While these are usually hosted externally and not part of the HTML file size itself, their collective download size can impact loading times, user experience, and indirectly, deliverability. For optimal email deliverability, the total email size (including text, images, and other elements) is generally recommended to not exceed 100KB. This also helps avoid spam trap triggers.
Excessively large emails can also lead to slower loading times, which impacts user experience and may cause recipients to abandon your message. This negative engagement can be interpreted by ISPs as a sign of low-quality content, further contributing to poorer inbox placement.
Code quality and spam filters
Beyond sheer size, the actual quality of your email's HTML and CSS code can significantly influence how spam filters perceive your message. Spam filters are sophisticated systems that analyze numerous factors, and messy or unconventional code can be a red flag. For instance, code generated by some WYSIWYG editors might include unnecessary tags, inline styling, or deprecated attributes that contribute to code bloat and potential issues.
Spam filters (and blacklists) look for patterns associated with malicious or unsolicited emails. If your email code contains errors, uses unsupported techniques, or appears obscure, it could trigger these filters. This isn't always about the code being heavy per se, but rather about its structure potentially mimicking code seen in spam. Mailbox providers err on the side of caution, and anything that looks suspicious can lead to filtering.
Conversely, clean, well-structured HTML and CSS are more easily parsed by email clients and spam filters alike. When filters can clearly understand and render your email's content, it reduces the likelihood of it being flagged as suspicious. We have found that avoiding HTML errors, unsupported techniques, and obscure content are key tips to lower your spam score.
While HTML bloat from WYSIWYG editors isn't inherently a direct cause of deliverability issues, the quality of that code matters. Missing alt tags, unclosed tags, or overly complex nested tables can all contribute to a higher spam score. Ensuring your HTML is valid and adheres to email coding best practices can significantly improve your chances of reaching the inbox.
Optimizing your email code
Optimizing your email code and content is a proactive step towards better deliverability. The goal is to create lean, clean, and efficient emails that load quickly and are easily interpreted by all email clients and spam filters. Here are some actionable strategies:
Minify HTML and CSS: Remove unnecessary characters, comments, and whitespace from your code. This reduces file size without affecting functionality. While minification is not obfuscation, it makes your code more compact.
Optimize images: Compress images without sacrificing quality and ensure they are hosted on reliable servers. Always include alt text for accessibility and as a fallback if images don't load. For more information, read our article, How do email image sizes affect deliverability?.
Inline CSS: While internal or external stylesheets are common in web development, inline CSS is generally preferred for emails to ensure consistent rendering across various clients.
Validate HTML: Use a reliable HTML validator to catch and fix any errors or warnings in your code.
Even with these best practices, sometimes auto-generated code from third-party tools can still be a bit heavier than hand-coded HTML. The key is to ensure that while it might be bloated, it is at least clean and free of errors that could raise spam filter suspicions. Regularly audit your email code to maintain optimal deliverability.
Views from the trenches
Best practices
Always minify your HTML and CSS to reduce overall email size and improve loading speed for recipients.
Ensure all images have proper alt tags and are optimized for the web, balancing quality with file size.
Test your emails across various email clients and devices to catch rendering issues and unexpected truncation.
Prioritize clean, semantic HTML over overly complex or nested table structures, which can be seen as suspicious.
Keep your email content concise and to the point, avoiding unnecessary elements or excessive blank spaces.
Common pitfalls
Relying solely on WYSIWYG editors without reviewing the generated code for bloat or errors.
Ignoring email clipping warnings from providers like Gmail, which indicate your email is too large.
Using unsupported HTML or CSS properties that may not render correctly and could trigger spam filters.
Embedding large, unoptimized images or media directly into the email, increasing total load time.
Failing to regularly test your email code quality, leading to unnoticed deliverability issues over time.
Expert tips
Leverage tools specifically designed for email HTML cleanup and minification to streamline your workflow.
Focus on the core message and call to action to prevent over-designing emails that become too heavy.
Consider the image to text ratio, as too many images compared to text can negatively affect deliverability.
Monitor your email deliverability rates closely, as a drop might signal underlying code or size issues.
Invest in professional email development if your campaigns require complex layouts to ensure optimal code.
Expert view
An expert from Email Geeks says that email deliverability is impacted by avoiding HTML errors, unsupported techniques, obscure content, and obfuscation within the email code.
2023-03-10 - Email Geeks
Expert view
An expert from Email Geeks highlighted that the issue with heavy email code isn't necessarily the weight itself, but that such code might be associated with spammer practices, leading to higher spam filter scrutiny.
2023-04-22 - Email Geeks
Crafting for the inbox
Email code quality and size are fundamental, yet often overlooked, aspects of email deliverability. While they might not be the primary drivers of spam filtering in every case, their impact is undeniable. Large email sizes can lead to clipping, frustrate recipients, and signal to mailbox providers that your content might be low quality or poorly constructed.
Similarly, messy or erroneous HTML code can trigger spam filters (or blocklists) that look for suspicious patterns. Prioritizing clean, lean, and validated code ensures your emails are easy to read for both humans and machines, thereby increasing their chances of landing in the inbox. Ultimately, by paying attention to these technical details, you are building a more robust foundation for your email program.
Gmail, have specific size thresholds, and exceeding these can lead to messages being truncated (clipped) or even rejected.
