Every email service limits how large a message can be, and the limit surprises people because it is smaller than the file they think they are sending. A 20 MB file may be rejected by a service whose limit is 25 MB, because the attachment grows when it is encoded for transport. A message that leaves your outbox may also bounce at the recipient's end, whose limit may be lower than yours. This guide explains what the attachment size limits are for Gmail, Outlook, Yahoo and iCloud as published at the time of writing, why the effective limit is lower than the headline number, what happens when a file is too large, how to send large files properly with links, compression and file-sharing features, how to fix a bounce caused by size and how application developers should handle attachments in transactional email. Limits change, so confirm the current figure with your provider.
Email attachment size limits by provider
| Service | Sending limit (approx.) | Notes |
|---|---|---|
| Gmail | 25 MB per message | Larger files are added through Google Drive as a link; messages received can be larger, up to 50 MB |
| Outlook.com (personal) | 20 MB per message in common configurations | Larger files can be shared through OneDrive links |
| Microsoft 365 / Exchange Online | 20 MB by default for sending, adjustable by administrators up to a higher ceiling | Depends on the organisation's settings |
| Yahoo Mail | 25 MB per message | Cloud links are suggested for larger files |
| iCloud Mail | 20 MB per message | Mail Drop can send much larger files through a download link |
These figures are published defaults at the time of writing and may differ for business accounts, older plans or mailboxes managed by an administrator. The only dependable answer is the current help page of your provider or the error message you receive.
Why the real limit is lower than the number
Email was built for text. To carry binary files such as images, PDFs and spreadsheets, attachments are converted into a text-safe form, most commonly Base64 encoding, and wrapped in MIME parts. Base64 makes the data about one third larger. A 20 MB file therefore becomes roughly 27 MB in the message, which exceeds a 25 MB limit. As a rule of thumb, keep attachments to around three quarters of the stated limit, and less if several files are attached. For how the packaging works, see MIME email attachments explained. The message body, signatures, embedded images and headers add to the total as well.
The recipient's limit matters too
Your provider may accept a 25 MB message while the recipient's company accepts only 10 MB. If your message is too large for the receiving server, it is rejected and you receive a bounce notice, often with a code starting with 5.2.3 or 5.3.4, or with wording such as "message too large" or "message size exceeds fixed maximum". Because you cannot know the recipient's limit, the safe approach for files above a few megabytes is to share a link instead of an attachment. For more on bounce codes and how to read them, see SMTP bounce codes explained and email bounce-back reasons.
What happens when an attachment is too large
- Your mail app stops you. Gmail, Outlook and others show a warning or automatically convert the file into a cloud link.
- The message stays in the outbox. A slow upload or a rejected size leaves it unsent.
- A bounce arrives. The recipient's server refuses the message, and you get a notice.
- Silent loss is rare but possible if a filter strips or quarantines large or risky files, which is why important files should be confirmed by a short message.
How to send a large file by email
1. Use the cloud link feature of your provider
Gmail offers Google Drive, Outlook offers OneDrive and iCloud offers Mail Drop. When you attach a file beyond the limit, the app uploads it and inserts a download link. Check the sharing permissions: choose the narrowest setting that works, such as only the people you name, and an expiry date for sensitive files.
2. Use a file-sharing service
Upload the file to a business storage or transfer service and share the link. Prefer a service that supports passwords and link expiry. Do not use an unknown site for confidential documents.
3. Compress the file
A ZIP archive can reduce documents, though images, videos and many PDFs are already compressed and shrink little. Do not rename a compressed file to hide its type; many filters block archives with executable content. Instead, reduce the original file: lower the resolution of images, export a PDF with a smaller size setting or save a video at a lower resolution.
4. Split the attachment
If you must email it, send several messages, each below the limit. This is a last resort and annoying for the recipient.
5. Use a secure portal for sensitive material
For contracts, medical or financial records, upload to a secure portal rather than email. See email encryption for the risks of sending such files by ordinary email.
When in doubt, ask the recipient what they can receive. A quick question before sending a large file saves both sides a failed delivery and a wasted upload.
Tips to keep messages small
- Link to images in newsletters instead of embedding large pictures, and compress them first. See newsletter design for image sizing.
- Remove unneeded high-resolution photos from documents before attaching.
- Keep signatures light; a huge logo repeated in every reply fills mailboxes. See HTML email signature.
- Clean up long reply chains with attachments in every message.
- Tell recipients what you sent and how to download it, so a link in a message is not mistaken for phishing.
How to fix a "message too large" bounce
- Read the bounce notice and note the size limit if it is stated.
- Check the real size of your message, including the encoding overhead.
- Replace the attachment with a link, or compress or split the file.
- Resend and confirm with the recipient.
- If the recipient is a company that often rejects large files, ask whether they have a preferred transfer method.
For developers: attachments in application email
If your application sends invoices, tickets or reports by email, consider the limits carefully. Attaching a file to every transactional message increases the size, the sending time and the chance of rejection or spam filtering. A cleaner pattern is to send a short message with a signed, expiring download link to the document, and to attach only small files. If you do attach files, enforce a maximum size in your code, use correct content types, name files sensibly and test with several providers. You can submit messages through our SMTP relay or the email API; check the documentation for the current message size limits that apply to your account. Also remember that inbound messages with attachments have limits; see inbound email processing.
Send documents without bouncing Send a short message with a secure download link instead of a heavy attachment, and watch delivery in your dashboard.
References and further reading
Frequently asked questions
What is the Gmail attachment size limit?
Gmail allows about 25 MB per message when sending, and larger files are shared as Google Drive links. Messages received can be somewhat larger. Check Google's current help page.
What is the Outlook attachment size limit?
Outlook.com commonly allows about 20 MB per message, and Microsoft 365 business mailboxes often default to 20 MB with administrator-adjustable limits.
Why is my file rejected even though it is under the limit?
Attachments are Base64 encoded, which adds about a third to their size, and the message body and headers add more. The recipient's limit may also be lower than yours.
How do I send a file larger than the limit?
Use your provider's cloud link feature, a file-sharing service with expiry and passwords, compress or reduce the file, or use a secure portal for sensitive documents.
What does a message too large bounce mean?
The recipient's server refused the message because it exceeds its size limit. Replace the attachment with a link or shrink the file and resend.