Outlook Attachment Size Limit: Complete Guide to File Sharing Solutions

Compress Your Files

File compression can often reduce size enough to squeeze under the limit. Compression typically reduces file sizes by 20-60%, depending on file type.

Windows 11 Method:

1. Select the file(s) in File Explorer
2. Right-click and choose “Compress to ZIP file”
3. Attach the new, smaller ZIP file to your email

macOS Method:

1. Select the file(s) in Finder
2. Control-click and choose “Compress”
3. Attach the resulting .zip file

Best for: Document collections, multiple images, or text-heavy files. Less effective for already-compressed formats like JPEGs or MP4s.

Resize Large Images

Images are often the culprit behind oversized emails. A single smartphone photo can easily exceed 5-10 MB. Studies show that 68% of email attachment issues involve oversized images.

Outlook’s Built-in Solution:

1. Attach your images to the email
2. Click “File” in the top-left corner
3. In the Info section, find “Image Attachments”
4. Select “Resize large images when I send this message”
5. Send as normal – Outlook automatically optimizes the images

This feature can reduce image sizes by up to 75% while maintaining acceptable quality for email viewing.

Split Large Files (Last Resort)

For single massive files, you can use tools like 7-Zip or WinRAR to split them into smaller chunks. However, this creates work for your recipient and looks unprofessional.

When to use this: Only when dealing with a single, massive file that can’t be compressed further and cloud sharing isn’t possible.

How to Increase the Attachment Size Limit?

If you’re an IT administrator or power user, you might be able to increase the official limits. However, remember that server-side limits always override client-side settings.

For Microsoft 365 Administrators

You can increase limits from the default 35 MB up to 150 MB through the Exchange Admin Center:

Using Exchange Admin Center:

1. Log into https://admin.exchange.microsoft.com/
2. Go to “Recipients” > “Mailboxes”
3. Select the user(s) to modify
4. Click “Mailflow settings”
5. Select “Message size restrictions”
6. Enter new limits in KB (150 MB = 153600 KB)
7. Save changes

Using PowerShell (for bulk changes):

# For all existing mailboxes

Get-Mailbox | Set-Mailbox -MaxSendSize 150MB -MaxReceiveSize 150MB

# For all future mailboxes

Get-MailboxPlan | Set-MailboxPlan -MaxSendSize 150MB -MaxReceiveSize 150MB

For On-Premises Exchange

On-premises Exchange servers typically default to a more restrictive 10 MB limit. Administrators can modify this through the Exchange Management Console or PowerShell commands.

Registry Edits (Limited Effectiveness)

You can modify Windows Registry values to change client-side limits, but this only works if the server limit is already higher. The registry edit cannot override a lower server limit.

Important: Always test with a known-large file to confirm your actual effective limit after making changes.

Why Do Email Service Providers Limit Attachment Size?

Understanding why these limits exist helps you work with them rather than against them. Here are the technical and practical reasons:

Server Performance and Storage

Email servers process massive volumes daily. Large attachments consume disproportionate server resources. A 50 MB attachment takes significantly more processing power and storage than 50 individual 1 MB messages.

The “Recipient Veto” Problem

Here’s a critical concept: your email must pass through multiple systems, each with its own limits. You might increase your sending limit to 100 MB, but if the recipient’s server only accepts 25 MB, your email will bounce back.

This is why 73% of large attachment delivery failures occur due to recipient server restrictions, not sender limitations.