Summary
Email file size and MIME types are crucial, yet often overlooked, factors influencing email deliverability. While modern email clients and internet speeds have advanced, the fundamental principles regarding email bloat and how it's transmitted remain highly relevant. Understanding the intricacies of how email code quality and size impact deliverability can significantly affect inbox placement and user engagement.
Key findings
- Size limit impact: The widely cited 100KB email file size limit, originating from a 2014 Email on Acid study, largely holds true today, particularly for major mailbox providers like Gmail. Exceeding this limit often leads to email clipping, where content is truncated and a link to view the full message is displayed. While not strictly a deliverability failure (the email still reaches the inbox), clipping negatively impacts user experience and potentially prevents users from seeing critical calls to action or tracking pixels.
- On-the-wire size: Email size is measured on-the-wire, meaning it includes all components of the email, such as headers, plain text versions, HTML content, and attachments. MIME (Multipurpose Internet Mail Extensions) encoding, which converts various data types into a format suitable for email transmission, adds to this size, typically increasing the total data by 20-30%.
- MIME type aggregation: The 100KB limit applies to the combined total of all MIME types within an email (e.g., HTML, plain text, and AMP). Sending an email with a 60KB HTML part and a 50KB AMP part would result in a total size of 110KB (plus encoding overhead), exceeding the threshold and increasing the likelihood of clipping or other issues. This is especially relevant for emails with multiple content types or large embedded media.
- AMP specific issues: While AMP emails offer interactive experiences, some providers, notably Outlook (for @outlook and @hotmail addresses), may experience reduced deliverability when receiving the AMP part. This suggests that certain MIME types might trigger specific filtering behaviors or rendering challenges with particular mailbox providers.
- Open tracking implications: When emails are clipped, the open tracking pixel, which is often placed at the end of the email, may not load. This leads to underreported open rates, which marketers might misinterpret as a deliverability problem when the email actually reached the inbox successfully. This highlights the ongoing challenge of accurate metric reporting in email marketing, especially as email clients evolve.
Key considerations
- Monitor email size: Regularly check the total size of your emails, including all MIME parts and attachments, to ensure they remain below the 100KB threshold. Tools that report on-the-wire size are most accurate.
- Optimize content: Minimize unnecessary HTML code, inline CSS, and large images to keep the overall email size down. Consider optimizing image sizes and formats, and self-hosting images when possible, as this affects the final email payload.
- Strategic AMP usage: If using AMP for Email, segment your audience and consider sending the HTML-only version to mailbox providers known to have deliverability issues with AMP (like Outlook). This ensures broader inbox reach even if it means sacrificing some interactivity for specific segments.
- Attachment best practices: Avoid large attachments where possible. Instead of directly attaching files, host them online and provide a link within the email. This reduces the email's overall size and avoids potential spam triggers associated with certain file types. Learn more about PDF attachment best practices.
- Focus on engagement: While clipped emails might show lower open rates, the primary goal should be to get the email into the inbox. Encourage engagement with compelling content above the fold, regardless of potential clipping. Focus on metrics beyond image-based opens, such as clicks and conversions.
What email marketers say
Email marketers frequently encounter challenges related to email file size, particularly concerning how it impacts their perceived deliverability and campaign performance metrics. The fear of emails being clipped by clients like Gmail creates stress, as it can hide crucial content and interfere with open tracking. Marketers often grapple with balancing rich, engaging content with the need to keep email sizes lean to avoid these issues. The discussion extends to the practicalities of how email service providers (ESPs) handle email construction and tracking, highlighting a tension between ideal tracking placement and preventing rendering problems for the diverse email client ecosystem.
Key opinions
- Clipping concerns: Many marketers are highly concerned about Gmail clipping emails longer than 100KB, as this impacts how subscribers interact with their messages and can obscure important calls to action at the bottom of the email. They perceive this as a significant user experience hurdle.
- Misinterpreted metrics: Clipped emails often lead to misreported open rates because the tracking pixel, typically at the email's footer, isn't loaded. Marketers might incorrectly assume a deliverability problem when the email actually landed in the inbox. This highlights a need for marketers to understand the nuances of why their deliverability rates might appear incorrect.
- AMP deliverability for outlook: Some marketers have observed a noticeable reduction in deliverability specifically when sending emails with an AMP part to Outlook or Hotmail addresses, leading them to adapt their sending strategies for these particular domains.
- ESPs and tracking pixels: There's a frustration among marketers that ESPs don't place open tracking pixels at the beginning of the HTML to ensure they fire even if the email is clipped. This seems like a simple fix to some, despite potential technical complexities.
Key considerations
- Prioritize critical content: Ensure your most important messaging and calls to action are placed at the top of the email, above the potential clipping point. This guarantees visibility even if the email exceeds the size limit. Think about how your content structure impacts deliverability.
- Optimize assets: Compress images, minimize inline CSS, and simplify HTML structure to reduce overall email file size. Every kilobyte counts towards staying under the clipping threshold.
- Segment for AMP: If using AMP for Email, consider audience segmentation. Send the HTML-only version to Outlook and Hotmail users to mitigate potential deliverability issues, ensuring all subscribers receive a well-rendered, deliverable email.
- Alternative engagement metrics: Given the unreliability of open rates for clipped emails, focus on alternative engagement metrics such as click-through rates, conversions, and website traffic originating from email campaigns. This provides a more accurate picture of campaign success.
- Educate your team: Ensure that marketing teams understand that email clipping is primarily a user experience and reporting issue, not necessarily a deliverability problem that indicates emails are going to spam. This can reduce undue stress and lead to more effective strategy adjustments. For more details, explore why emails fail and how to fix it.
Marketer view
An email marketer from Email Geeks suggests that marketers often worry unnecessarily about clipped emails. They believe that while it impacts reporting, the email still reaches the inbox, and the core deliverability isn't compromised. The stress caused by misinterpreting clipped emails as delivery failures is a significant pain point for many in the industry.
Marketer view
An email marketer from EmailLabs.io explains that large attachments significantly increase email size, which can lead to deliverability problems. They emphasize that while attachments are a common feature, their size must be carefully managed to prevent emails from being blocked or delayed by recipient servers. Exceeding size limits is a frequent cause of delivery issues.
What the experts say
Deliverability experts acknowledge that while the core principle of keeping email file sizes lean remains relevant, the direct impact on spam filtering might have evolved. They distinguish between clipping (a user experience issue) and outright deliverability failure (emails going to spam or being rejected). Experts emphasize that modern spam filters are sophisticated and consider a multitude of factors beyond just size, though excessive bloat can still be a contributing negative signal. The discussion also touches on the technical complexities faced by ESPs in balancing flexible tracking solutions with robust email rendering across diverse client environments.
Key opinions
- Clipping is not spam: Experts generally agree that emails being clipped by Gmail (at ~102KB) is a user experience issue, not a spam or deliverability problem. The email still reaches the inbox; it's merely truncated. This distinction is vital for accurate diagnosis of inbox placement issues.
- Size as a contributing factor: While filters have advanced, excessive email size can still be a negative signal, especially if combined with other suspicious characteristics. It's less about a hard cutoff for spam and more about contributing to an overall poor sender reputation score. This highlights the importance of understanding email domain reputation.
- On-the-wire size: The relevant size measurement is the on-the-wire size, which includes all email components, headers, and the MIME encoding overhead. This encoding can add a significant percentage to the raw content size.
- ESP pixel placement logic: ESPs typically place tracking pixels at the end of the email's HTML body to minimize the risk of rendering breaks. Placing them at the beginning carries a higher risk of visibly broken images or layout issues if the pixel fails to load or interacts negatively with CSS, especially given the crude injection methods some ESPs use. This explains why they prioritize rendering stability over perfect open tracking.
- Evolution of filters: While the 2014 Email on Acid data was foundational, experts believe filters have significantly improved since then and are less reliant solely on email size for spam detection. Modern filters analyze a broader range of signals, including engagement, sender reputation, and authentication status. Understanding these signals can help to boost email deliverability rates.
Key considerations
- Differentiate clipping from spam: Educate internal teams that Gmail clipping is a user experience and reporting challenge, not a deliverability failure. This clarity helps in focusing efforts on actual inbox placement issues rather than misdiagnosed problems.
- Account for MIME encoding: When calculating email size, remember the 20-30% overhead from MIME encoding. A 60KB HTML email will be larger on-the-wire than its raw HTML size, impacting the effective threshold. Also consider how different encoding methods affect deliverability.
- Holistic deliverability view: Don't rely solely on email size as a deliverability indicator. Focus on a comprehensive deliverability strategy that includes strong sender reputation, proper authentication (SPF, DKIM, DMARC), and consistent engagement. This broad approach provides a more robust defense against spam filtering. Consider using a free online email testing tool to check your email's health.
- Optimized content strategy: Prioritize a concise and impactful message for the visible portion of clipped emails. Design for an effective preview experience, ensuring that key information and a clear call to action are immediately apparent without requiring the user to click view entire message.
- Technical trade-offs: Understand that ESPs make design choices for tracking pixel placement to balance accurate data with the risk of breaking email rendering. While not ideal for open tracking on clipped emails, this approach prioritizes reliable display for the majority of recipients.
Expert view
An expert from Email Geeks, who is a co-founder of Email on Acid, confirms that emails exceeding 100KB are frequently clipped in Gmail. They clarify that this is primarily a rendering issue for the recipient, not an indicator of spam or a failure in email deliverability. The email still reaches the inbox, but the user experience is impacted, and parts of the message might be hidden until expanded. They advise focusing on the on-the-wire size, which accounts for all components including headers.
Expert view
An expert from SpamResource.com indicates that while very large emails can be viewed suspiciously by filters, it is rarely the sole factor determining inbox placement. Modern spam filters are far more complex, evaluating sender reputation, content relevance, user engagement, and authentication records (like SPF, DKIM, and DMARC). They suggest that size typically becomes an issue only when combined with other negative signals. Therefore, senders should prioritize overall email health over a rigid focus on size alone.
What the documentation says
Official documentation and research on email standards, though sometimes dated, offer foundational insights into how email size and MIME types operate. They explain the technical necessity of MIME encoding for transmitting diverse content beyond plain text, inherently increasing the 'on-the-wire' size. While RFCs define how email should be constructed, practical limitations and individual mailbox provider policies often dictate what actually gets delivered and displayed effectively. These documents highlight that email attachments and complex content types necessitate size management to ensure reliable delivery and avoid hitting system limits.
Key findings
- MIME encoding overhead: MIME (Multipurpose Internet Mail Extensions) encoding is fundamental for sending anything other than plain text, including HTML, images, and attachments. This encoding process typically adds 20-30% to the original data size, increasing the 'on-the-wire' footprint of the email. This overhead is a necessary component of modern email communication.
- Universal MIME support: MIME is the backbone of contemporary email, enabling the transmission of rich media. Without it, emails would be limited to plain text. Its universal adoption allows for complex, visually engaging emails, but it comes with the trade-off of increased file size due to the encoding process.
- Attachment size limits: Despite MIME's ability to support large files, many email clients and servers impose their own limitations on attachment sizes. Exceeding these limits can lead to delivery failures, as mail providers aim to conserve storage and bandwidth. These limits can vary significantly between providers.
- Impact on transfer speeds: Large email files, especially those with numerous or bulky attachments, require more network resources and bandwidth for transfer. This can slow down email delivery, affecting the timeliness of communications and potentially burdening both sender and recipient systems. This is an inherent consequence of increased data payload.
- Client-specific rendering: Different email clients and servers support various file types and rendering capabilities. Choosing the correct MIME types and optimizing content ensures messages are displayed as intended, preventing rendering issues that might impact how a message is consumed or even if it's readable. This relates to what RFC 5322 says versus what actually works in practice.
Key considerations
- Mind the encoding increase: Always account for the additional size incurred by MIME encoding when designing emails, especially those with rich HTML or attachments. The raw file size is not the final 'on-the-wire' size.
- Respect attachment limits: Adhere to commonly accepted email attachment size limits (e.g., typically 20-25MB for major providers), or better yet, link to external files instead of attaching them to avoid rejection or delays. This is crucial for avoiding spam trap triggers.
- Optimize content types: Choose efficient MIME types for your content. For example, optimize images for web use (compressed JPEGs, PNGs) rather than high-resolution print files to minimize size. Consider the impact of image-only emails and their size.
- Prioritize load speed: While internet speeds have improved, large email files still take longer to download and render. Optimize HTML and CSS to ensure quick loading, providing a better user experience and potentially increasing engagement. A fast loading email is less likely to be abandoned by impatient recipients.
- Fallback content: Always include a robust plain-text version (a separate MIME part) of your email. This ensures readability even if the HTML fails to render or if the recipient's client blocks certain content types, providing a reliable fallback for all users.
Technical article
Quora explains that all email content, including attachments, undergoes MIME encoding. This process converts various data types into a format compatible with email transmission. Consequently, the encoded data size typically increases by 20-30% compared to the original file size. This inherent overhead means that even small attachments contribute to a larger overall email footprint on-the-wire. Understanding this increase is crucial for managing total email size.
Technical article
Mutant Mail documentation highlights MIME as the essential foundation for modern email communication. It enables the sending of diverse media types—including images, videos, and other complex data—beyond simple plain text. Without MIME, email functionality would be severely limited, underscoring its critical role in today's rich, multimedia-driven email experience. This mechanism is what allows email to be more than just text-based messages.
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