> ## Documentation Index
> Fetch the complete documentation index at: https://docs.mailofly.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Unsupported Attachment Types

> File type restrictions, size limits, and security policies for email attachments.

# Unsupported Attachment Types

To prevent malware propagation and ensure high inbox delivery rates, Mailofly restricts certain executable, script, and high-risk attachment types.

***

## Blocked File Extensions

The following file extensions are strictly blocked by Mailofly and major receiving mail transfer agents (Gmail, Outlook, Yahoo):

| Category            | Blocked Extensions                                              |
| :------------------ | :-------------------------------------------------------------- |
| **Executables**     | `.exe`, `.dll`, `.bin`, `.msi`, `.dmg`, `.iso`, `.com`          |
| **Scripts & Code**  | `.bat`, `.cmd`, `.sh`, `.vbs`, `.js`, `.jse`, `.ps1`, `.scr`    |
| **System Archives** | Password-protected or encrypted archives containing executables |

If an email payload includes a blocked file type, the API rejects the request with an HTTP 400 error.

***

## Recommended Formats

* **Documents**: `.pdf`, `.docx`, `.xlsx`, `.pptx`, `.csv`, `.txt`
* **Images**: `.png`, `.jpg`, `.jpeg`, `.gif`, `.webp`, `.svg`
* **Archives**: Standard `.zip` and `.tar.gz` (containing safe documents)
* **Calendar**: `.ics`

***

## Size Limits

* **Maximum Total Payload**: 25 MB per email (including headers, HTML content, and base64-encoded attachments).
* **Base64 Overhead**: Base64 encoding inflates binary file size by approximately **33%**. For example, a raw 18 MB file will encode to \~24 MB.
* **Large Files**: For attachments exceeding 20 MB, we recommend hosting the file (e.g. AWS S3, Cloudflare R2, Google Cloud Storage) and providing a secure download link in the email body.


## Related topics

- [Attachments](/sending/attachments.md)
- [Retrieve Attachment](/api/receiving/retrieve-attachment.md)
- [List Attachments](/api/receiving/list-attachments.md)