Summary
Email file size and MIME types significantly affect email deliverability and user experience. Exceeding email provider size limits (25MB for Gmail, 150MB for Exchange Online) results in rejection. Emails over 100KB often get clipped in Gmail, affecting the user experience and potentially obscuring tracking pixels or calls-to-action. Larger emails are more likely to be flagged as spam. Correct MIME types are crucial for proper rendering and avoiding spam filters. Tracking pixels, especially if placed at the top, can cause rendering issues. Optimizing images, using efficient HTML code, validating MIME types, and testing emails are vital for ensuring deliverability. Sending AMP emails to Outlook can reduce deliverability.
Key findings
- Size Limits: Gmail has a 25MB limit, and Exchange Online has a 150MB limit; exceeding these results in rejection.
- Gmail Clipping: Emails exceeding 100KB are often clipped in Gmail, impacting user experience.
- Spam Risk: Larger email sizes increase the likelihood of being flagged as spam.
- MIME Importance: Correct MIME types ensure proper email rendering and prevent spam flagging.
- Tracking Pixel Issues: Improperly placed tracking pixels can break email rendering.
- Outlook AMP Issue: Sending AMP emails to Outlook can reduce deliverability.
Key considerations
- Optimize Size: Optimize images and HTML code to minimize email file size.
- Validate MIME: Ensure correct MIME types are used for all email content.
- Test Emails: Test emails before sending to large lists to ensure proper rendering and file size.
- Pixel Placement: Carefully consider the placement of tracking pixels to avoid rendering issues.
- Limit AMP usage for Outlook: Avoid sending AMP emails to Outlook and send HTML emails instead.
What email marketers say
9 marketer opinions
Email file size and MIME types significantly impact email deliverability. Large emails can trigger spam filters, cause clipping in email clients (like Gmail), and lead to slow loading times and poor engagement, especially on mobile devices. Correct MIME types are crucial for proper rendering and avoiding misinterpretation by email clients. Keeping HTML file sizes under 100KB and optimizing images, code, and attachments can improve deliverability and ensure the entire message is displayed. Additionally, using Google App Script to preview total size is useful. Sending AMP parts to Outlook can reduce deliverability so only sending the HTML part may be required.
Key opinions
- Size Limit: Keeping HTML file sizes under 100KB can avoid clipping in Gmail and improve overall deliverability.
- MIME Types: Using correct MIME types ensures proper rendering across different email clients and prevents emails from being flagged as spam.
- Mobile Impact: Large emails can lead to slow loading times and poor engagement on mobile devices.
- Spam Filters: Oversized emails and unoptimized code can trigger spam filters, reducing deliverability.
- Outlook Issues: Sending AMP emails to Outlook can reduce deliverability and may require to send only the HTML emails.
Key considerations
- Optimization: Optimize images, minify code, and avoid unnecessary attachments to reduce email file size.
- Testing: Test emails before sending them to large contact lists to preview the total size and ensure proper rendering.
- Mobile Responsiveness: Ensure emails are mobile-friendly and load quickly to improve engagement.
- MIME Validation: Always validate MIME types to prevent rendering issues and avoid spam filters.
- Client Specific Issues: Be aware that client specific issues can arise with different email clients requiring different MIME types and email builds.
Marketer view
Email marketer from Email Geeks shares that deliverability is considerably reduced when an AMP part is sent to Outlook. To mitigate this, they only send the HTML part to users with an @outlook or @hotmail address.
21 Aug 2022 - Email Geeks
Marketer view
Email marketer from StackOverflow user MailGuru explains that using correct MIME types ensures that email clients properly render the email content. Incorrect MIME types can cause emails to display incorrectly or be flagged as spam.
11 Mar 2025 - StackOverflow
What the experts say
7 expert opinions
Email file size and MIME types significantly impact deliverability and user experience. Emails exceeding 100kb are often clipped in Gmail, affecting customer experience. Larger emails can be flagged as spam by ISPs and email clients. Correct MIME types are crucial for proper rendering. Tracking pixels, if not handled correctly, can also cause rendering issues, especially if placed at the top of the email. Optimizing images, using efficient HTML, and ensuring proper MIME formatting are crucial for avoiding deliverability problems.
Key opinions
- Email Clipping: Emails larger than 100kb are often clipped in Gmail, impacting user experience and potentially missing critical content like tracking pixels.
- Spam Filtering: Larger email sizes increase the likelihood of being flagged as spam by ISPs and email clients.
- MIME Type Importance: Incorrect MIME types can cause display issues and trigger spam filters, negatively affecting deliverability.
- Tracking Pixel Risks: Improperly placed or handled tracking pixels can break email rendering, leading to broken images or layout issues.
- Header Size: Email size includes the header size, which contributes to the total size.
Key considerations
- File Size Optimization: Optimize images and use efficient HTML code to minimize email file size and avoid triggering spam filters.
- MIME Type Validation: Ensure emails use the proper MIME types for the content they contain to prevent display issues.
- Tracking Pixel Placement: Consider using templating setups to safely place tracking pixels, avoiding potential rendering issues.
- Use of aboutmy.email: Utilizing tools like aboutmy.email is useful to show the actual size of the email, including headers.
Expert view
Expert from Email Geeks explains that when mail is clipped, the bottom isn’t rendered, which often contains the open tracking pixel, leading to misinterpretation as a delivery issue.
1 Sep 2023 - Email Geeks
Expert view
Expert from Email Geeks shares a case where overly simplistic CSS media queries blew up a pixel from 1x1 to 300x300. He explains ESPs add tracking pixels at the end of emails to avoid issues and reduce customer service calls.
22 May 2022 - Email Geeks
What the documentation says
4 technical articles
Email file size and MIME types significantly affect email deliverability as highlighted by official documentation. Gmail has a message size limit of 25 MB, while Exchange Online has a limit of 150 MB. Exceeding these limits will result in the email being rejected. Proper MIME encoding, as specified by IETF and RFC Editor, is crucial for email clients to correctly interpret and render email content, preventing misinterpretation and spam flagging.
Key findings
- Gmail Size Limit: Gmail's message size limit is 25 MB, including attachments and inline images.
- Exchange Online Size Limit: Exchange Online's message size limit is 150 MB, and exceeding this will lead to rejection.
- MIME Importance: Correct MIME encoding is essential for ensuring emails are correctly interpreted by email clients.
- MIME Standards: RFC Editor outlines the standards for Internet Message Format, including MIME types, for proper email rendering.
Key considerations
- Size Adherence: Adhere to the message size limits set by email providers like Gmail and Exchange Online to ensure delivery.
- MIME Configuration: Ensure proper MIME encoding for all email content to prevent misinterpretation and spam flagging.
- Attachment Size: Consider the size of attachments and inline images when composing emails to stay within size limits.
- Email Validation: Validate that email content is correctly constructed and follows relevant standards.
Technical article
Documentation from Google Workspace Admin Help specifies that Gmail has a message size limit of 25 MB. This includes all attachments and inline images. Exceeding this limit may result in the email being rejected or undelivered.
17 Apr 2022 - Google Workspace Admin Help
Technical article
Documentation from Microsoft states that Exchange Online has a maximum message size limit of 150 MB. Emails exceeding this limit will be rejected. Understanding and adhering to these limits is crucial for ensuring email deliverability within the Exchange Online environment.
21 Nov 2021 - Microsoft
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