---
title: Sending Fax via API
description: "--- This guide explains how to send outbound faxes using the eFax API. It includes the API endpoint, authentication requirements, request parameters, request and response examples, error handling, and troubleshooting information. > **Prerequisite** > > > > Before using the…"
url: "https://www.dataphone.cloud/docs/sending-fax-via-api/"
updated: "2026-06-24"
category: API
---

# Sending Fax via API

---

This guide explains how to send outbound faxes using the eFax API. It includes the API endpoint, authentication requirements, request parameters, request and response examples, error handling, and troubleshooting information.

> **Prerequisite**
>
>
>
> Before using the eFax API, ensure you have a valid **API Key**.
>
>
>
> API keys are issued by the **Tech Support Team**.
>
>
>
> If you do not already have an API key, please contact Tech Support and follow the guide below for instructions on requesting one:

---

# API Endpoint

```
POST https://api.documo.com/v1/faxes
```

---

# HTTP Method

```
POST
```

---

# Authentication

The API uses **Basic Authentication**.

Include your API key in the `Authorization` header.

### Example

```
Authorization: Basic YOUR_API_KEY
```

> Replace **YOUR_API_KEY** with the API key generated from your eFax account.

---

# Request Headers

| Header | Value |
| --- | --- |
| Authorization | Basic YOUR_API_KEY |
| Content-Type | multipart/form-data |

---

# Request Body

The request body must be sent as **multipart/form-data**.

## Required Parameters

| Parameter | Type | Description |
| --- | --- | --- |
| faxNumber | String | Destination fax number including country code (Example: 12125551234). |
| attachments | File | File(s) to be faxed. Required unless using a cover page only. |

---

## Optional Parameters

| Parameter | Type | Description | Example |
| --- | --- | --- | --- |
| coverPage | Boolean | Includes the default cover page. | true/false |
| recipientName | String | Recipient name displayed on the cover page. | John Smith |
| senderName | String | Sender name displayed on the cover page. | ABC Company |
| subject | String | Subject shown on the cover page. | Invoice 10025 |
| notes | String | Notes displayed on the cover page. | Please review and sign. |
| callerId | String | Outbound fax number to send from. If omitted, the account default is used. | 12125550000 |
| scheduledDate | ISO Date | Schedule the fax for a future date/time. | 2026-06-25T15:30:00Z |
| async | Boolean | Returns immediately with a Message ID instead of waiting for processing. | true |
| optimizeFax | String | Fax optimization mode. Supported values: **auto**, **text**, **image**. | auto |

---

# Example Request

### URL

```
POST https://api.documo.com/v1/faxes
```

### Headers

```
Authorization: Basic YOUR_API_KEY
Content-Type: multipart/form-data
```

### Body

| Key | Type | Value |
| --- | --- | --- |
| faxNumber | Text | 12125551234 |
| recipientName | Text | John Smith |
| senderName | Text | ABC Company |
| subject | Text | Invoice #10025 |
| callerId | Text | 12125550000 |
| notes | Text | Please review and sign the attached invoice. |
| async | Text | true |
| optimizeFax | Text | auto |
| attachments | File | invoice.pdf |

---

# Successful Response

When the request is accepted, the API returns a response similar to:

```
{
    "messageId": "f3b0c5d7-8c29-4f4c-a3de-xxxxxxxxxxxx",
    "status": "Processing"
}
```

The **messageId** uniquely identifies the fax submission and can be used to track its status.

---

# Error Handling

## 400 Bad Request

Returned when required parameters are missing or invalid.

Example:

```
{
    "error": {
        "message": "faxNumber is required"
    }
}
```

---

## 401 Unauthorized

Returned when authentication fails.

Example:

```
{
    "error": {
        "name": "UnauthorizedError",
        "message": "Wrong authorization type"
    }
}
```

Verify that:

- The API key is valid.
- The Authorization header uses:

```
Authorization: Basic YOUR_API_KEY
```

---

## 403 Forbidden

The authenticated account does not have permission to perform the requested action.

---

## 404 Not Found

The requested API endpoint is incorrect or unavailable.

---

## 422 Unprocessable Entity

Returned when one or more request parameters are invalid.

Examples include:

- Invalid fax number.
- Unsupported file format.
- Attachment exceeds the maximum file size.
- Invalid Caller ID.

---

## 500 Internal Server Error

An unexpected server-side error occurred.

Retry the request after a few moments. If the issue persists, contact support.

---

# Troubleshooting

## Authentication Fails

Verify that the Authorization header is configured as:

```
Authorization: Basic YOUR_API_KEY
```

Do **not** use:

```
Authorization: Bearer YOUR_API_KEY
```

or

```
Authorization: Key YOUR_API_KEY
```

---

## Invalid Fax Number

Ensure the fax number:

- Includes the country code.
- Contains only digits.
- Does not include spaces, brackets, or hyphens.

**Correct Example**

```
12125551234
```

---

## Attachment Not Accepted

Verify that:

- The attachment is a supported document type.
- The file size is within the API limits.
- The attachment is uploaded as a **File** using **multipart/form-data**.

---

## Wrong Caller ID

If multiple outbound fax numbers exist on your account, specify the desired sending fax number using the `callerId` parameter.

If omitted, the account's default outbound fax number will be used.

---

## Request Times Out

For applications that do not need to wait for processing to complete, enable asynchronous processing:

```
async=true
```

The API will immediately return a **messageId**, allowing the application to continue while the fax is processed in the background.

---