How does email file size and MIME types affect email deliverability?
Matthew Whittaker
Co-founder & CTO, Suped
Published 30 Apr 2025
Updated 17 Aug 2025
8 min read
Email deliverability is a complex ecosystem, and while many factors influence whether your messages land in the inbox, the physical size of your email and how its content is structured (MIME types) play a more nuanced role than you might initially think.
In an age of high-speed internet, it might seem counterintuitive that something like file size still matters. However, email servers and clients often operate with specific limitations and processing behaviors that can turn an oversized or improperly formatted email into a deliverability challenge, even if it eventually reaches the recipient.
The implications range from minor annoyances like content clipping to more severe issues such as increased bounce rates or messages being diverted to the spam folder. Understanding these dynamics is crucial for anyone aiming for optimal email marketing and communication.
The impact of email file size on deliverability
The total size of an email, encompassing its HTML code, plain text version, embedded images, and attachments, directly impacts how email service providers (ESPs) and internet service providers (ISPs) process it. A widely discussed benchmark in the industry, initially highlighted by Email On Acid, suggests that HTML email files over 100KB can increase the likelihood of deliverability issues. This threshold remains largely relevant today, despite advancements in internet speeds.
One of the most common consequences of exceeding this size limit is email clipping, particularly by clients like Gmail. When an email's HTML body exceeds approximately 102KB, Gmail will clip the message, displaying only the initial portion and requiring the recipient to click a View entire message link. While this doesn't prevent the email from reaching the inbox, it can significantly impact user experience and engagement. Crucially, if your open tracking pixel is at the end of the email, clipped messages can lead to underreported open rates, skewing your campaign analytics.
Beyond clipping, large email sizes can strain recipient servers, leading to slower processing times or even rejection. Email servers often have file size limits for incoming messages, and exceeding these can result in bounces. Moreover, spam filters may view excessively large emails, especially those bloated with hidden code or overly complex structures, as suspicious. This can trigger a blocklist (or blacklist) action or result in placement directly into the spam folder, negatively affecting your sender reputation.
Attachments are a primary culprit for increasing email size. When you attach a file, it's typically encoded using Base64, which inflates its size by approximately one-third. So, a 10MB PDF could become over 13MB on the wire, pushing total email size beyond common limits (which can range from 10MB to 50MB across different providers). Consider alternative methods for sharing large files, such as linking to cloud storage.
MIME types and their role in email delivery
MIME, or Multipurpose Internet Mail Extensions, is a standard that allows emails to include different types of content beyond simple plain text, such as HTML, images, audio, video, and application-specific files. Each distinct part of an email, like its HTML version, plain text version, or an attachment, is defined by its own MIME type.
While MIME is essential for rich email experiences, the combination and complexity of these parts can affect deliverability. For example, a multipart email containing both HTML and plain text (a common practice for fallback) adds to the total file size. Adding an AMP HTML part further increases this on-the-wire size.
Certain MIME types, especially those indicating executable content or less common file formats, might be viewed with suspicion by security-conscious mail servers. For instance, while PDF attachments are common, sending them can sometimes negatively impact deliverability, as some filters may flag them as potential security risks. Similarly, while GIFs can enhance engagement, their file size needs careful management.
AMP HTML emails and outlook
Although Google is a strong proponent of AMP for email, some mail providers, particularly Outlook, have shown reduced deliverability for emails containing an AMP part. This isn't universally true but is a notable observation.
To mitigate this, some senders choose to segment their audience, sending only the HTML version to @outlook.com or @hotmail.com addresses. This proactive approach can help maintain high deliverability rates across different email clients.
Strategies for optimizing email size and content
Optimizing your email's file size and MIME structure is a critical step in ensuring your messages land in the inbox without issues. Here are practical strategies:
Image optimization: Always compress images before embedding them. Use appropriate formats like JPG for photos and PNG for graphics with transparency. Consider hosting images externally and referencing them via URL rather than embedding them via Base64, which increases email size significantly.
HTML and CSS minification: Remove unnecessary whitespace, comments, and redundant code from your HTML and CSS. Every character counts. Inlining CSS is often recommended for email compatibility, but ensure it's concise. Learn more about email code quality.
Attachment best practices: Avoid attaching large files directly. Instead, upload them to a cloud service and include a link in your email. This significantly reduces the email's size and can prevent it from being blocked due to attachment limits. For more information, see our guide on PDF attachments.
Image-to-text ratio: While less of a deliverability concern than in the past, a disproportionately high image count can still increase overall email size. Aim for a balanced design that prioritizes content while being visually appealing. Read more about the image to text ratio.
Regularly testing your email size can provide valuable insights. Many online tools allow you to paste your HTML and check its weight. Keeping an eye on this can prevent unexpected clipping or delivery issues.
Impact on metrics and sender reputation
The effects of email file size and MIME types extend beyond mere delivery, influencing how your email campaigns perform and how your sending domain is perceived by ISPs. The direct impact on open ratesdue to clipping is significant; if your tracking pixel is not loaded, your reported open rates will be inaccurately low. This can lead to incorrect conclusions about campaign effectiveness and audience engagement.
Furthermore, consistently sending oversized emails or messages with problematic MIME structures can subtly, or sometimes overtly, harm your sender reputation. ISPs monitor various signals, and emails that consistently trigger server load issues, bounce more often, or contain suspicious elements might be flagged. Over time, this can lead to your emails being directed to the spam folder or even your domain being added to a blocklist (or blacklist).
Recipient engagement is a key factor in deliverability. If emails are slow to load, clipped, or frustrating to interact with due to size, recipients are more likely to delete them, mark them as spam, or simply ignore them. These negative engagement signals tell ISPs that your content is not valuable, which can further degrade your sender reputation and lead to future delivery issues.
Views from the trenches
Best practices
Keep your HTML content under 100KB to prevent clipping, especially in Gmail, which ensures the full message is rendered.
Host large images and attachments externally on cloud storage and include links, rather than embedding them directly in the email.
Always include a plain text version of your email. This provides a fallback for clients that don't render HTML and improves deliverability.
Regularly audit your email templates for unnecessary code, hidden elements, or bloated CSS that could be increasing file size.
Common pitfalls
Ignoring Gmail's 102KB clipping limit, which can hide critical content and open tracking pixels, leading to inaccurate metrics.
Embedding all images via Base64, which significantly inflates email size and can trigger spam filters.
Sending large attachments directly in emails, which often leads to bounces or delays due to recipient server limits.
Using overly complex or poorly optimized HTML and CSS, adding unnecessary weight to the email file.
Expert tips
For AMP emails, consider conditional sending: deliver the AMP version only to recipients where it's known to render reliably.
Prioritize mobile responsiveness in your design, as compact, efficient code tends to perform better on smaller screens and load faster.
Use email testing tools to preview how your emails render across different clients and check their total file size.
Focus on the quality and value of your content; a well-crafted, concise email often performs better than a large, convoluted one.
Expert view
Expert from Email Geeks says emails longer than 100KB are often clipped in Gmail, which primarily affects rendering and open tracking rather than direct deliverability to the inbox.
2024-09-16 - Email Geeks
Expert view
Expert from Email Geeks says when mail is clipped, the open tracking pixel, usually at the bottom, is often not rendered, causing a misinterpretation of delivery issues.
2024-09-16 - Email Geeks
Final thoughts on email optimization
The size and MIME structure of your emails are critical, yet often overlooked, components of deliverability. While direct server rejection due to file size might be less common than it once was, the indirect impacts on user experience, open tracking, and ultimately, sender reputation, are substantial. Email clipping by major providers and the nuanced handling of different MIME types (like AMP HTML by some clients) highlight the need for careful optimization.
By adhering to best practices such as keeping HTML concise, optimizing images, minimizing attachments, and understanding how different MIME parts are interpreted, you can significantly improve your chances of reaching the inbox and ensuring your messages are fully seen and engaged with by your audience. A thoughtful approach to email construction is an investment in your overall email program's success.
Gmail. When an email's HTML body exceeds approximately 102KB, Gmail will clip the message, displaying only the initial portion and requiring the recipient to click a View entire message link. While this doesn't prevent the email from reaching the inbox, it can significantly impact user experience and engagement. Crucially, if your open tracking pixel is at the end of the email, clipped messages can lead to underreported open rates, skewing your campaign analytics.
Google is a strong proponent of AMP for email, some mail providers, particularly 
Outlook, have shown reduced deliverability for emails containing an AMP part. This isn't universally true but is a notable observation.
