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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:

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API Endpoint #

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

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HTTP Method #

POST

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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.

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Request Headers #

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

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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.

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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

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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

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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.

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Error Handling #

400 Bad Request #

Returned when required parameters are missing or invalid.

Example:

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

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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

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403 Forbidden #

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

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404 Not Found #

The requested API endpoint is incorrect or unavailable.

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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.

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500 Internal Server Error #

An unexpected server-side error occurred.

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

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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

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Invalid Fax Number #

Ensure the fax number:

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

Correct Example

12125551234

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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.

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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.

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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.

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