Outlook Attachment Size Limit: How to Send Large Files

Outlook Attachment Size Limits

Important: These limits apply to the TOTAL size of all attachments combined, not per file. A 15 MB file + a 10 MB file = 25 MB total, which may exceed the limit on some Outlook versions.

Also note: The receiving server may have its own limits. Even if Outlook lets you send 25 MB, the recipient's email system might reject anything over 10 MB.

Method 1: Share via OneDrive (Best Option)

Microsoft's recommended solution for large files:

From Outlook Desktop

1. Click New Email
2. Click Attach File → select your file
3. If the file exceeds the limit, Outlook automatically prompts: "Upload to OneDrive"
4. Click Upload — the file uploads to OneDrive
5. A shareable link is inserted in your email instead of the attachment
6. Send the email — recipients click the link to download

From Outlook Web

1. Compose a new email
2. Click the paperclip icon → Browse this computer
3. Select your file
4. Choose "Upload and share as a OneDrive link" (for files over 20 MB, this is automatic)
5. Set permissions (anyone with link, or specific people)
6. Send

Advantages of OneDrive Links

• Files up to 250 GB can be shared
• You control permissions (view only, edit, download)
• You can revoke access after they download
• No attachment size conflicts with the receiving server
• Version control — if you update the file, they see the latest

Method 2: Compress the File

If the file is slightly over the limit, compression might bring it under:

Windows

1. Right-click the file → Send to → Compressed (zipped) folder
2. Attach the .zip file instead

Mac

1. Right-click the file → Compress [filename]
2. Attach the .zip file

Expected Compression Rates

If compression doesn't get you under the limit, use OneDrive/cloud links instead.

Method 3: Google Drive / Dropbox / Other Cloud Storage

Not everyone uses OneDrive. Other options:

Google Drive

1. Upload your file to drive.google.com
2. Right-click → Share → Get link
3. Set permissions to "Anyone with the link"
4. Paste the link in your Outlook email

Dropbox

1. Upload to Dropbox
2. Click Share → Copy link
3. Paste in your email

WeTransfer

1. Go to wetransfer.com
2. Upload file (up to 2 GB free)
3. Enter your email and recipient's email
4. WeTransfer sends the download link directly

Method 4: Split the File

For very large files that can't be cloud-shared (restricted environments):

Split a ZIP File (Windows — 7-Zip)

1. Right-click file → 7-Zip → Add to archive
2. Set "Split to volumes" to 20 MB (or 20000000 bytes)
3. 7-Zip creates multiple parts: file.zip.001, file.zip.002, etc.
4. Send each part in a separate email
5. Recipient saves all parts to one folder and extracts

Limitation: Confusing for non-technical recipients. Use cloud links if possible.

Method 5: Resize Images and Videos

If you're sending media files that are too large:

Images

• Outlook built-in: When attaching large images, Outlook offers "Resize to fit email"
• Manual: Use an image editor to reduce resolution (1920px wide is usually sufficient for email)
• Batch resize: Tools like ImageOptim (Mac) or TinyPNG (web)

Videos

• Don't email videos — they're almost always too large
• Upload to OneDrive/Google Drive and share a link
• Use a video platform (Loom, YouTube unlisted, Vimeo)
• Compress with HandBrake (free) if you must attach

Method 6: Change Outlook's Attachment Limit (Admin)

For Exchange/Microsoft 365 administrators who need to increase the limit:

Exchange Online (Microsoft 365 Admin)

The maximum configurable limit is 150 MB for Exchange Online:

1. Go to Exchange Admin Center
2. Navigate to Mail flow → Rules
3. Create a transport rule OR
4. Use PowerShell:

Set-Mailbox -Identity user@company.com -MaxSendSize 50MB -MaxReceiveSize 50MB

For organization-wide:

Set-TransportConfig -MaxSendSize 50MB -MaxReceiveSize 50MB

Note: Increasing limits can cause problems — large attachments slow servers and fill mailboxes quickly. Cloud links are the better long-term solution.

What Happens When You Exceed the Limit

The worst case: Your email appears to send successfully but bounces back hours later. Always check your inbox for bounce notifications after sending large attachments.

Best Practices for Attachments

1. Use cloud links by default for anything over 5 MB
2. Name files descriptively — "Q2-Report-April-2026.pdf" not "doc1.pdf"
3. Compress before attaching — it's a courtesy
4. Mention the attachment in your email body — "Attached is the [file]" prevents confusion
5. Don't send more than 3 attachments — use a ZIP or cloud folder
6. Use PDF for documents — universal compatibility
7. Test with a colleague first for critical large files

Sending Files in Cold Email

For cold outreach, attachment rules are different:

• Never attach files in the first cold email — triggers spam filters
• Links to hosted content are better (but even links increase spam scores)
• Offer to send rather than attaching: "I have a case study that's relevant — want me to send it over?"
• If you must share files, use a link to a landing page or hosted PDF

ColdRelay provides cold email infrastructure where deliverability is the priority:

• $1 per mailbox — purpose-built for outbound
• Deliverability-first — no unnecessary attachments, clean sends
• SPF, DKIM, DMARC — authentication handled
• Best practice guidance — infrastructure designed for how cold email actually works

Keep your cold emails clean, short, and attachment-free. Save the files for after they reply.

FAQ

Can I increase Outlook.com's 20 MB limit?

No. Outlook.com (personal accounts) has a fixed 20 MB limit that can't be changed. Use OneDrive links for larger files.

Does compressing a PDF reduce quality?

Zipping a PDF doesn't change its quality — it's lossless compression. However, using PDF compression tools (like Adobe's "Reduce File Size") may slightly reduce image quality within the PDF.

Can the recipient download OneDrive files without a Microsoft account?

Yes — if you set permissions to "Anyone with the link can view/download." They don't need a Microsoft account.

Why did my 20 MB file get rejected even though the limit is 25 MB?

Email encoding (MIME/Base64) increases file size by ~33%. A 20 MB file becomes ~26.7 MB after encoding. The actual limit for the raw file is roughly 19 MB for a 25 MB email size limit.

Is there a way to auto-compress attachments in Outlook?

Not built-in. Some add-ins (like AutoZip) can automatically compress attachments before sending. For images specifically, Outlook offers resize on attach.

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Don't let attachment limits slow you down. And when you're sending outreach, skip attachments entirely. ColdRelay — clean cold email infrastructure at $1/mailbox.