Microsoft Outlook is one of the most widely used email platforms in both corporate and personal settings. As digital file sizes continue to grow and the demand for sharing high-resolution images, videos, and documents increases, understanding Outlook’s email attachment limitations is critical. Whether you’re sending a project to a colleague or submitting a large report to a client, knowing how big your email attachments can be helps ensure seamless communication.
TL;DR
The default maximum attachment size in Microsoft Outlook is 20 MB for most versions when used with Microsoft Exchange or Office 365. This limit can vary based on how Outlook is configured or the type of email server it connects to. Workarounds like OneDrive and file compression can help you send larger files. Always check with your administrator if you’re using a corporate account, as custom policies may override defaults.
Why Attachment Size Limits Exist
Email was never designed to send large files directly. Back when internet connections were slower and storage was more expensive, email systems were set up with constraints to manage data traffic and prevent overload. Even today, attachment size limits are necessary for managing:
- Server performance
- Network bandwidth
- Email delivery reliability
- Security and virus scanning
While modern technologies have eased many of these limitations, email services like Outlook still impose certain attachment size restrictions by default. These limits can differ depending on whether you’re using Outlook with a Microsoft Exchange Server, Outlook.com, Office 365, or another email service.
Default Maximum Attachment Size in Outlook
Microsoft Outlook, depending on how it is configured and the email server it uses, permits attachments of different sizes.
Outlook with Microsoft Exchange or Office 365
When Outlook is used with Microsoft Exchange Server or Microsoft 365 (previously Office 365), the default maximum attachment size is:
- 20 MB for sending emails
- 25 MB for receiving emails
The actual values can be modified by your system administrator using Exchange Admin Center or through PowerShell scripts. However, unless changes have been made, the 20 MB limit is standard, particularly in corporate environments.
Outlook.com (formerly Hotmail)
When you’re using Outlook.com manually through a browser or through the mail client, your attachment size limit is slightly larger:
- 34 MB per email if you attach files directly
- Up to 2 GB per file when sharing via OneDrive integration
This makes Outlook.com a more flexible option for personal users who need to occasionally send larger files.
Mail Server Considerations
Even if your Outlook client allows you to attach 20 MB files, keep in mind that:
- The recipient’s email service may have stricter limits
- Intermediary servers and gateways may reject large attachments
- Email attachments can become slightly larger due to Base64 encoding, which increases file size by about 33%
For example, attaching a 19 MB file from Outlook might exceed the limit because the encoded message exceeds the 25 MB threshold after conversion. This encoding is required for sending binary files within Ethernet protocols.
Customizing Attachment Settings
Advanced users and IT administrators can change the attachment size limit via the Windows Registry or via administrative consoles like the Exchange Admin Center.
Modifying via Windows Registry (Client-Side Outlook)
For Outlook versions installed on Windows computers, the attachment limit can be increased or decreased using a simple Registry edit. This is particularly useful if you are not connected to Exchange.
- Open the Windows Registry Editor (
regedit) - Navigate to:
HKEY_CURRENT_USER\Software\Microsoft\Office\[version]\Outlook\Preferences - Create or modify a DWORD value:
MaximumAttachmentSize - Set its value in KB (e.g., 51200 for 50 MB)
Important: Editing the registry can cause serious system problems if done incorrectly. Always back up the registry before making changes.
Modifying via Exchange Admin Center
For enterprise users, administrators can set organizational limits in Exchange to define both send and receive size restrictions. These settings can override Outlook client configurations.
Consult your IT department or Microsoft’s official documentation for instructions tailored to your infrastructure.
Alternatives to Sending Large Attachments
If your file sizes exceed the default limits in Outlook, consider the following workarounds:
1. Use OneDrive or SharePoint
Microsoft recommends storing large files in OneDrive and sharing a link instead of attaching the file directly. Outlook integrates seamlessly with OneDrive, enabling this with just a few clicks.
- Upload the file to OneDrive
- Click “Insert” > “Share Link” in Outlook
- The recipient gets a secure download link
2. Compress Files
Use a tool like WinRAR, 7-Zip, or Windows’ built-in compression utility to compress files into ZIP or RAR formats. This can reduce file size enough to slide under the attachment limit.
3. Split Files
Larger files can be split into smaller parts using archiving tools. These can then be sent in separate emails and reassembled by the recipient.
4. Use Third-Party File Transfer Services
Services like WeTransfer, Dropbox, or Google Drive allow you to host large files and share download links. These tools are particularly helpful when dealing with recipients on different platforms.
Common Error Messages Related to Attachment Sizes
Outlook provides specific error messages when your email exceeds the allowed size. Common alerts include:
- “The attachment size exceeds the allowable limit”
- “This message couldn’t be sent. Try reducing the file size or sending fewer attachments.”
- “Your message has not been sent because the maximum size was exceeded.”
These errors are a clear indication that the built-in limits have been reached. To resolve them, consider using the alternative methods outlined above.
Real-World Use Cases and Recommendations
In corporate settings, employees often need to send high-resolution slide decks, videos, or datasets. Using internal shared drives or enterprise solutions like Microsoft Teams or SharePoint reduces the need for direct attachments. On the other hand, personal users might find workarounds like ZIP files or cloud links more practical and accessible.
For freelancers and remote workers, combining Outlook with OneDrive is particularly effective. Not only does it simplify file management, but it also ensures better compliance with size limitations and data policies.
Conclusion
Understanding the attachment size limits in Microsoft Outlook is essential for maintaining effective communication and avoiding unnecessary frustration. While the default limits may seem restrictive, multiple solutions exist to work around them. Whether you are an everyday user or an IT administrator, knowing your configuration and available tools will help you work more efficiently.
Always evaluate whether attaching a file is essential, or if sharing a link offers a more secure, flexible, and size-friendly approach. By adapting your file-sharing methods, you can stay within Outlook’s limitations while maintaining the professionalism and speed required in today’s workflow.
