Outlook & Exchange Attachment Size Limits Explained
Microsoft Outlook and Office 365 default to a 20 MB attachment cap for standard accounts and 34 MB for Exchange Online. Attempting to attach larger files triggers the infamous 'The file you're attaching is bigger than the server allows' error.
Up to 500 files or folders · Client-side AES-256-GCM encryption · Zero accounts required
Start Free TransferOutlook & Microsoft 365 limit breakdown
| Constraint | Details |
|---|---|
| Stated attachment limit | 20 MB (free Outlook.com / Hotmail), 34 MB (Microsoft 365 Enterprise) |
| Encoding overhead | Exchange MIME encoding adds 33% bloat |
| Cloud fallback behavior | Prompts to share via OneDrive link |
| Recipient requirement | External Exchange servers often cap at 10 MB or 20 MB |
| Privacy & scanning | Scanned by Microsoft Defender; unencrypted at rest on recipient server |
Why files bounce on Outlook & Microsoft 365
Enterprise mail administrators routinely set maximum incoming message sizes between 10 MB and 25 MB to prevent mailbox bloat and DDoS. Even if your corporate Outlook allows sending 34 MB, the recipient's mail server will bounce it back with error 552 5.3.4 (message size exceeds limit).
How to send files larger than 20 MB – 34 MB
- Use LimeShare to generate an end-to-end encrypted transfer link up to 3 GB.
- Paste the link into your Outlook email body — it avoids all mail server attachment limits.
- Set a password (Argon2id) and send it via Microsoft Teams or SMS for dual-channel enterprise security.
Frequently asked questions
Why does Outlook say 'The file you're attaching is bigger than the server allows'?
Your Exchange administrator has set a maximum attachment size rule (typically 20 MB). Files larger than this cannot be attached directly to the message.
How can I email a 1 GB file through Outlook?
You cannot attach a 1 GB file directly. Use LimeShare to encrypt the 1 GB file client-side and paste the self-destructing link into your Outlook email.