What are the attachment and message size limits for different mailbox providers?

Key findings

• Total message size: Mailbox providers impose limits on the total size of an email, encompassing both the message body and all attachments. This is not solely an attachment limit.
• Encoding overhead: Attachments are typically encoded (such as base64), which adds to their size. A 10 MB file could become approximately 13.4 MB on the wire.
• Varying limits: Limits differ significantly across providers and even within the same provider depending on the client (web vs. desktop) or user tier. For most email providers, the maximum message size is typically between 10 to 25 MB. EmailLabs provides a good overview.
• Common thresholds: While some providers allow up to 150 MB for specific clients, most general limits are much lower, often around 25 MB for common services like Gmail and Yahoo Mail.

Key considerations

• Impact on deliverability: Large email sizes can affect deliverability, potentially leading to delays, throttling, or messages being rejected or routed to spam folders.
• User experience: Sending very large emails can be slow for recipients, especially those with limited bandwidth or storage.
• Alternative sharing: For files exceeding common limits or for better deliverability, consider using cloud storage services and sharing links instead of direct attachments. This is also advised for PDF attachments.
• Proactive monitoring: Regularly check message sizes and monitor bounces related to size limits to maintain optimal deliverability.

Key opinions

• Unpredictability: Marketers find it challenging to keep track of varying message size limits across different mailbox providers, leading to guesswork.
• Preference for cloud links: Many prefer to link to files stored in cloud services (e.g., Google Drive, Dropbox) rather than attaching them directly, as this bypasses size restrictions and potentially improves deliverability.
• Deliverability concerns: There's a common understanding that sending large emails can negatively impact inbox placement and lead to higher bounce rates, particularly when considering common connection and message limits.
• Recipient experience: Slow download times or outright rejections for large attachments can frustrate recipients and detract from the user experience.

Key considerations

• Optimize content: Prioritize efficient email design and content, using image compression and minimal attachments to stay within limits.
• Fallback strategies: Always have a plan for sharing larger files, such as linking to hosted versions, to avoid email delivery failures.
• User education: Educate internal teams and senders about email size best practices to avoid common pitfalls.
• Monitoring and testing: Regularly test email delivery with attachments to understand how different providers handle them, especially when mailbox quotas might be tight.
• Campaign considerations: For email marketing campaigns, it is often best practice to avoid direct attachments altogether to maximize deliverability and engagement, as highlighted by GrowthList's insights on email sending limits.

Key opinions

• Message size, not attachment size: Experts emphasize that the limit applies to the entire email message's payload as it travels over the wire, not just the raw attachment file size.
• Encoding matters: Attachments are often MIME-encoded (e.g., base64), which can increase their size by approximately 34% when transmitted.
• How to check limits: Tools like swaks can be used to query a mail server and read the SIZE extension in the EHLO response, which indicates the maximum acceptable message size in bytes.
• Modern minimums: Limits lower than 10 megabytes are rarely encountered in contemporary email systems, according to expert observations.

Key considerations

• Accurate calculation: When planning emails with attachments, it's crucial to calculate the total encoded message size to avoid exceeding limits.
• Protocol understanding: A deeper understanding of SMTP (Simple Mail Transfer Protocol) and its extensions, particularly the SIZE parameter, provides clarity on how these limits function. This is part of general technical considerations for preventing delays.
• Vendor specifics: While general trends exist, specific providers like Microsoft (for Office 365) have unique ways of handling email volume and message limits. Practical 365 offers insights into configuring these.

Key findings

• Gmail: Typically allows attachments up to 25 MB in total message size. This limit applies to both sent and received emails.
• Outlook/Microsoft 365: Limits vary widely. For web interfaces (OWA), it's often around 20-35 MB. For desktop Outlook and Microsoft 365 paid subscriptions, it can be as high as 150 MB. Ablebits.com provides a comprehensive guide on this.
• Yahoo Mail: Generally has a limit around 25 MB per email, though some sources indicate exceptions for files attached via cloud services.
• iCloud Mail: Often cited with a 20 MB message size limit.

Key considerations

• Check official sources: Always refer to the latest official documentation from each mailbox provider for the most accurate and up-to-date limits, as these can change.
• Client-specific limits: Be aware that limits can vary depending on whether the email is sent or received via a web client, desktop client, or mobile app.
• Administrative control: For business accounts (e.g., Microsoft 365), administrators often have the ability to adjust these limits up to a certain maximum. Understanding how Microsoft Exchange receiving limits work is key.
• Recipient server limits: The lowest limit between the sending and receiving server will dictate the maximum allowed message size. It's not just about your sender's capabilities but also the recipient's. This relates to common connection and message limits.