What is email size?

Email size refers to the total amount of data in an email, including:

• Message body (text or HTML)
• Attachments (images, PDFs, etc.)
• Headers (sender, receiver, subject, metadata)
• Embedded images or formatting (if using rich text or HTML)

Why does email size matter?

Email size plays a key role in whether your message gets delivered, how it renders, and how users interact with it. Most providers enforce a 25 MB limit (attachments and MIME encoding included). Anything over that gets rejected at the SMTP level.

Even if your email is under that limit, Gmail will clip the content if the raw HTML exceeds around 102 KB. When that happens, users see a “[Message clipped] View entire message” link, which can easily be missed.

Beyond that, large emails use more bandwidth, increase storage usage, and may trigger spam or security filters. On mobile or slower networks, they can load slowly or time out entirely—leading to a poor user experience and lower engagement.

How does email size affect deliverability?

Higher chance of spam filtering
Larger emails are more likely to get caught in spam filters—especially if they include big attachments, too many images, or bloated HTML. Many filters treat large messages as potential threats (like phishing or malware), which increases the risk of being flagged or blocked entirely.

Research from platforms like Email on Acid shows that emails between 15 KB and 100 KB tend to have the best deliverability. Once you go over 100 KB, the chances of hitting the inbox drop. Emails over 110 KB are especially risky, as multiple spam filters tend to flag them—even if they’re only slightly above that size.

Lower engagement
Larger emails take longer to load, especially on mobile devices or slower connections. If a message lags or doesn’t render quickly, users are more likely to skip it or delete it. That drop in engagement can hurt your sender reputation, which in turn affects deliverability.

Risk of delivery delays or bouncebacks
Very large emails can cause delays or queuing issues on sending servers. This can result in deferred delivery or even rejection by receiving servers with strict size limits. Exceeding these limits often leads to bouncebacks or failed deliveries.

Attachment size limit: The maximum size allowed per file attachment—or the sum of all attachments—in one message.

Total message size limit: Includes the body (plain text and HTML), attachments, and the extra bytes added by encoding (MIME overhead). Most servers enforce a cap on this “all-in” size.

HTML body size limit (Clipping): Some clients will truncate (clip) your message if the raw HTML exceeds a certain threshold, hiding the rest behind a “View entire message” link.

Header limits: Email headers store metadata like sender, recipient, subject, and routing info. According to Google’s guidelines, individual header fields (excluding certain ones like To, Cc, and References) are limited to 32 KB, and the total size of all headers combined should not exceed 500 KB. Additionally, each header line must be under 998 characters as per email formatting standards.

Email size caps can differ between free and paid plans, and each provider handles larger files in its own way:

Gmail (Google): Gmail allows users to send emails up to 25 MB and receive emails up to 50 MB, even on free accounts. If your attachment exceeds 25 MB, Gmail automatically uploads it to Google Drive and sends a download link instead. This behavior applies to both free and Google Workspace (paid) users, though Workspace users may benefit from larger Drive storage and admin-level controls.

Outlook.com (Microsoft): For Outlook.com (the free personal email service from Microsoft), the email size limit is 20 MB, including attachments. Larger files can be sent via OneDrive, which gets automatically suggested when attachments exceed the limit.

Exchange accounts, often used in businesses, have a default email size limit of 10 MB, which admins can increase up to 150 MB.

iCloud Mail (Apple): iCloud Mail users are limited to 20 MB per message. When a message exceeds your ISP's size limits, Mail Drop kicks in, allowing you to send attachments up to 5 GB. These attachments are hosted in iCloud for 30 days and don’t count against your iCloud storage quota. Mail Drop works with both free and paid iCloud accounts.

Yahoo Mail: Yahoo Mail enforces a 25 MB limit on the total size of all attached files in a single message, both for incoming and outgoing emails. This limit applies across free and Yahoo Mail Plus accounts. Users needing to send larger files must rely on third-party cloud storage services.

Proton Mail: Proton Mail, known for its privacy-first email service, enforces a 25 MB total message size limit on both free and paid plans.

Zoho Mail: Free Zoho Mail accounts support emails up to 20 MB in total size, including attachments. Paid plans unlock the Huge Attachment feature, which lets users send much larger files: 250 MB on Mail Lite, 500 MB on Workplace Basic, and up to 1 GB on Mail Premium and Workplace Professional. These are sent via secure download links instead of traditional attachments.

Mail.com: The maximum attachment size is 30 MB for Mail.com FreeMail users and 100 MB for Mail.com Premium users.

AOL Mail: Supports attachments up to 25 MB in size for both incoming and outgoing messages. This limit applies uniformly to all users, regardless of whether they’re using free or paid AOL services.

Attachments: Adding files like PDFs, Word docs, or presentations increases your email’s size—and not just by the file size itself. These attachments are Base64 encoded for safe transmission over email protocols, which adds about 33% overhead. So a 1 MB PDF becomes roughly 1.33 MB once encoded.

Embedded images: Inline images (like logos or product visuals) make emails visually appealing but also undergo Base64 encoding. That means they come with the same ~33% size increase. A single high-res image can easily add 200–500 KB or more to your total message size.

HTML and CSS complexity: Heavy use of tables, deeply nested

s, and lots of inline CSS can seriously inflate your email. Complex layouts not only increase size but may also cause rendering issues across clients. Keeping your markup clean and minimal helps reduce weight and improves compatibility.

Lengthy signatures and legal footers: A simple text signature is lightweight, but when you add social icons, embedded images, and legal disclaimers, it starts to bulk up. In long email threads, these repeated elements can pile on and significantly increase the total size.

Email headers and authentication data: Headers contain metadata like sender/recipient info, subject lines, and authentication details (SPF, DKIM, etc.). Individually, they’re small—but in forwarded or threaded emails, these can accumulate and add noticeable overhead.

Encryption overhead: Using S/MIME or PGP adds security, but it also increases the size of your emails. This happens because encryption wraps the original message in additional layers of headers, formatting, and encoded data. For instance, attachments are typically base64-encoded during encryption, which expands their size, and digital signatures or certificates further contribute to the overall growth. The exact increase depends on the encryption method and the structure of the email.

Fonts and web font requests: Embedding custom fonts or loading them via CSS can enhance design, but it’s easy to overlook how much size they add. Fonts can contribute hundreds of kilobytes, which affects load time and could impact deliverability—especially on mobile.

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testmail.app can help you catch size-related issues before sending. Test attachments, check HTML weight, and run spam score checks to improve deliverability.

How to send large files via email?

Compress or archive: Often, you can significantly reduce the size of your files by compressing them into a ZIP archive. On macOS, simply right-click the file or folder and select "Compress." Windows users can utilize tools like 7-Zip or WinRAR to create archives, frequently achieving a size reduction of 10–30%.

Split into chunks: For very large archives, consider splitting them into smaller, more manageable pieces. Applications such as 7-Zip offer the functionality to divide a large ZIP file into specified sizes, for example, creating five 50 MB parts. You can then send each of these smaller chunks as separate email attachments.

Use cloud storage: Use your preferred cloud storage service like Google Drive, Dropbox, or OneDrive. Upload the large file to your cloud drive and then generate a shareable link. Simply paste this link into your email, allowing recipients to download the file directly from the cloud.

Employ file-transfer services: Several free file-transfer services are designed specifically for large files. Platforms like WeTransfer enable you to upload files up to 2–5 GB and then email a download link to your recipients.

Host on your own server: If you maintain a website or have access to an FTP/SFTP server, you can upload the large file there and then share the URL via email. This approach is particularly useful for repeated sharing or when distributing files within an organization.

Best practices for lightweight, well-formatted emails

Keep email file size within 75–100 KB: Aim for a body size between 15 KB and 100 KB, ideally around 75 KB, to avoid Gmail clipping and reduce spam risk.

Avoid large attachments: Including large files as email attachments can sometimes trigger spam filters in certain email clients. Instead, it’s often better to host documents, images, or videos externally—on your website or a cloud service—and simply link to them.

Optimize images: It’s a good idea to resize images to match how they'll appear in the email—around 600 px wide for main content and 150 px tall for signatures usually works well. For formats, JPEG is a solid choice for photos, while PNG or WebP work better for graphics or logos. Try to avoid large file types like TIFF, which aren’t email-friendly. Hosting images externally and embedding them via HTML can help keep your emails lightweight. To support deliverability, aim for a balanced mix of visuals and text—something close to a 40:60 image-to-text ratio often performs better.

Use a single-column responsive layout: A single-column layout around 600–650 px wide tends to work best across most devices and email clients. Using responsive design techniques or tools can help ensure your emails look good on any screen without adding extra code.

Minimize HTML and CSS overhead: Try to write clean, minimal HTML—avoid overly complex structures, deep nesting, or unnecessary inline styles unless they’re really needed. Before sending, it's a good idea to minify your HTML and CSS to strip out whitespace and comments and reduce the overall size.

Use standard fonts and sizes: Using web-safe fonts in familiar sizes—like 12 pt for body text and 10–14 pt for headers—can help keep your emails lightweight and consistent across email clients, without needing to load custom fonts.

Limit content length and complexity: A good target height for an email is usually between 1500 and 2500 pixels. While longer emails can work, it's best to avoid going much beyond 3000 pixels to keep readers engaged. Try to keep your copy concise, with clear headings and calls to action. Instead of embedding videos or large GIFs, consider linking to them—and if you do use GIFs, aim to keep them under 500 KB.

Test email size and rendering: Before you hit send, it’s worth previewing your email on different devices (desktop, tablet, mobile) and in popular clients like Gmail, Outlook, and Apple Mail. This helps catch any formatting issues, misaligned elements, or missing images before your email goes out.