How do email file size and image hosting affect gmail deliverability?
Matthew Whittaker
Co-founder & CTO, Suped
Published 15 Jun 2025
Updated 18 Aug 2025
9 min read
Email deliverability is a complex challenge for many businesses, and getting your messages to the inbox, especially with Gmail being a dominant provider, can be particularly tricky. Two often-overlooked factors that significantly influence whether your emails land in the primary inbox or the spam folder are the overall email file size and how your images are hosted. I often see senders focusing on content and sender reputation, which are crucial, but overlooking these technical aspects can undermine all their efforts.
Understanding how Gmail's systems interact with these elements is vital for optimizing your email campaigns. It's not just about getting the email delivered, but ensuring it loads quickly and displays correctly, enhancing the recipient's experience and fostering positive engagement.
Email file size and deliverability: why it matters
One of the most immediate impacts of a large email file size is on loading times. When an email is excessively large, it takes longer to download, especially on mobile devices or slower internet connections. This can lead to a frustrating user experience, with recipients potentially abandoning the email before it fully loads. Moreover, large files can strain recipient mail servers, making them less likely to accept your emails, particularly during high-volume sending periods. This can result in emails being rejected or delayed, directly affecting your deliverability rates.
Gmail has a well-known limit for email size, often cited at 102KB. If your HTML email exceeds this threshold, Gmail will "clip" your message, displaying only the first portion and providing a link to "View entire message." While this doesn't directly prevent delivery, it significantly degrades the user experience and can hide important calls to action or content. It also signals to Gmail that your email might be less optimized, which could, over time, subtly affect your sender reputation. Many email service providers recommend keeping the overall email size under 100KB to avoid clipping and maximize inbox placement.
Beyond user experience, excessively large email files, particularly those with heavy HTML or numerous embedded assets, can trigger spam filters. These filters are designed to protect users from malicious content and server overload. An email that is unusually large might be flagged as suspicious, even if its content is legitimate. This is one of the reasons why some email service providers (ESPs) set their own maximum email sizes or recommend adhering to best practices to avoid deliverability issues.
To mitigate these risks, it is essential to optimize your email's HTML and CSS code, removing any unnecessary characters, comments, or redundant styling. Consider minimizing whitespace and inlining critical CSS. For attachments, instead of embedding them directly, it's often better to host them externally and provide a link within the email. This reduces the email's initial file size, making it lighter and faster to load, and less likely to hit file size limits imposed by recipient servers.
The 102KB rule for Gmail
Gmail will clip (truncate) any email with an HTML size exceeding 102KB. This means your recipients might not see your full message, leading to a poor experience and potentially missed calls to action. While it doesn't prevent delivery entirely, it can affect engagement and perceived quality, indirectly impacting future deliverability.
Impact on deliverability
Although not a direct block, consistent clipping can signal to Gmail that your emails are not well-optimized, which might lead to lower engagement metrics. Lower engagement can, in turn, negatively influence your sender reputation and inbox placement over time. It is a best practice to keep your email file size well below this limit.
The critical role of image hosting and domain reputation
Images are a crucial component of modern email marketing, but how they are included can drastically affect your deliverability. There are two primary methods: embedding images directly within the email (often using Base64 encoding or CID attachments) or hosting them externally on a server and linking to them. Embedding images increases the email's file size because the image data is part of the email itself. Conversely, externally hosting images keeps the email lighter, as the recipient's email client downloads the images from your server when the email is opened.
While hosting images externally helps with email file size, it introduces another critical factor: the reputation of the domain hosting your images. Gmail, like other major mailbox providers, scrutinizes every domain present in an email, not just the sending domain. This includes links to websites, tracking pixels, and, crucially, image hosting domains. If the domain hosting your images has a poor reputation, is new, or is associated with spam, it can negatively impact your email's deliverability, even if your primary sending domain is pristine. This is because all elements of the email contribute to its overall trustworthiness and legitimacy.
Many senders use Content Delivery Networks (CDNs) or cloud storage services like Amazon S3 for image hosting. While these are technically sound, the specific hostname used for image hosting can affect deliverability. Using a generic S3 bucket domain, for example, might not have the same positive reputation signals as a subdomain directly associated with your brand. Gmail's algorithms are sophisticated, and they consider the entire ecosystem of domains linked within an email. If you use a shared or generic domain for your images, it can be problematic if other senders using that same domain engage in spammy behavior, potentially getting your email caught in a blacklist (or blocklist).
To mitigate this, it's often recommended to use a subdomain of your primary sending domain for image hosting, such as images.yourdomain.com. This ensures that the reputation of your primary domain extends to your image hosting, reinforcing trust with mailbox providers. This practice aligns with Google's guidelines for bulk senders which emphasize consistent and reputable sending practices across all domains and IP addresses involved in your email stream.
Embedded images (inline/Base64)
File size impact: Significantly increases the total email file size, potentially leading to Gmail clipping.
Loading speed: Images load immediately because they are part of the email payload.
Deliverability: Less concern about external image host reputation, but high email size can trigger filters.
Offline viewing: Images are visible even without an internet connection after initial download.
Offline viewing: Images may not display if the recipient is offline.
Optimizing image content for inbox placement
Optimizing your images is paramount for email deliverability. This includes choosing the right file format (JPG for photos, PNG for graphics with transparency, GIF for animations), compressing them without significant loss of quality, and ensuring their dimensions are appropriate for email. Large image file sizes can slow down email loading times, frustrating recipients and potentially leading to lower engagement. Using modern compression techniques can significantly reduce file size while maintaining visual integrity, making your emails more efficient.
Beyond file size, consider the overall visual presentation. Ensure your images are responsive, adapting well to different screen sizes. More importantly, always include descriptive alt text for every image. Alt text serves multiple purposes: it improves accessibility for users with visual impairments, displays if images are blocked by email clients, and provides context for spam filters. A good alt text can even contribute positively to your email's content reputation.
Another crucial aspect is the image-to-text ratio. While modern spam filters are more sophisticated than in the past, an email composed almost entirely of images with minimal text can still be flagged. This is because spammers often use images to bypass text-based content filters. Maintaining a healthy balance, where text content is substantial and images complement the message rather than replacing it, is a key best practice for email deliverability and spam filtering.
Sending image-only emails (or emails with a very high image-to-text ratio) poses significant risks. Not only can they trigger spam filters due to their resemblance to spam techniques, but they also offer a poor user experience if images don't load. Without sufficient text, recipients are left with a blank or broken email, leading to frustration and potential unsubscribes. It is important to remember that image-only emails are generally bad for deliverability.
Aspect
Recommendation
Impact on deliverability
File size
Compress images to the smallest possible size without quality loss, ideally under 100KB per email.
Host images on a subdomain of your main sending domain (e.g., images.yourdomain.com).
Aligns image reputation with sender reputation, preventing blocklists (or blacklists) based on unknown image hosts.
Alt text
Always include descriptive alt text for all images.
Improves accessibility and provides content context to spam filters if images are blocked.
Ratio
Maintain a healthy text-to-image ratio, avoiding image-only emails.
Reduces chances of being flagged as spam due to image-heavy content, which spammers often use.
Views from the trenches
Best practices
Always aim for email file sizes under 100KB to prevent Gmail clipping and ensure faster loading.
Use a dedicated subdomain for hosting email images, such as images.yourdomain.com, to consolidate sender reputation.
Optimize all images for web use by compressing them and using appropriate formats.
Common pitfalls
Exceeding Gmail's 102KB limit, leading to message clipping and a poor user experience.
Using generic or shared domains (like default S3 buckets) for image hosting, which can have poor reputation.
Sending image-only emails or those with a very low text-to-image ratio, often flagged as spam.
Expert tips
Regularly monitor your email deliverability rates and engage with platforms like Google Postmaster Tools.
Implement DMARC, SPF, and DKIM to build strong email authentication and domain reputation.
Segment your audience and personalize content to boost engagement, which positively impacts deliverability.
Marketer view
Marketer from Email Geeks says they've encountered deliverability issues when their email file size exceeded 102KB, leading to clipping in Gmail and slower load times.
2019-08-14 - Email Geeks
Expert view
Expert from Email Geeks says that while HTML under 102KB avoids clipping, a significant impact on delivery occurs when large image file sizes are used, especially those over 100KB.
2019-08-14 - Email Geeks
Ensuring your emails reach the Gmail inbox
Email file size and image hosting are often underestimated elements of email deliverability, particularly when targeting Gmail's inbox. An oversized email risks clipping, slow loading, and triggering spam filters, while improperly hosted images can undermine your sender reputation. It's a holistic challenge that requires attention to detail, from the kilobytes of your HTML to the domain reputation of your image server. Addressing these technical nuances is as important as crafting compelling content and maintaining a clean list for long-term inbox success.
To ensure your emails consistently reach the inbox, focus on minimizing overall email size, optimizing all images, and carefully considering where and how your images are hosted. Regular testing and monitoring are essential to identify and rectify any issues before they impact your campaign performance. By proactively managing these technical factors, you can significantly enhance your email program's effectiveness and maintain a strong sending reputation with Gmail and other mailbox providers.
