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How can you integrate the fax API into an application?

Last updated : September 04, 2026

Fax is still a solution used today, but with a modern twist. To take advantage of this innovative option without having to manually send documents, which slows down the application, you can integrate the fax REST API into your application. How can a developer handle this?

• Fax API Integration
• Webhooks as a way to control delivery

Fax API Integration

The first step is configuring API access. The provider provides authentication data. The API key should be stored server-side, preferably as an environment variable or in a secure secret vault. It should not be directly contained in the application code or publicly shared.
After authentication, the application can send the document using the appropriate REST endpoint. Example APIs, such as mfax.to, allow the file to be transferred along with the recipient's number in an HTTP request.
First, create an API key in your provider's dashboard. This is needed to authenticate subsequent requests. Next, configure the HTTP client and provide the API address. Authorisation is provided as a Bearer token. The request must include the recipient's number and the document. The API uses form-data, allowing the file to be uploaded directly as part of the request. The fax number will be transmitted in the international E.164 format.

After successfully accepting the request, the API returns a response with the fax ID and its initial status. It's important to distinguish between request acceptance and delivery. Sending occurs asynchronously, and the request's status changes during execution.

Webhooks as a way to control delivery

The application still needs to receive information about what happened to the sent document. Webhooks are the best way to do this. A webhook is an endpoint provided by the application. The developer can prepare, for example, a webhooks/fax address and configure it in the service.

In the case of mfax.to, events related to the queue, start of sending, delivery, or transmission failure can be used. This eliminates the need for the application to constantly query the API for status. The webhook endpoint also serves to verify the authenticity of received requests.

The out-of-the-box integration allows for the creation of a simple flow: the application retrieves the document and recipient number, authenticates the request, passes the data to the API, stores the fax ID, and then waits for the result. The REST API is responsible for executing the transmission, and the webhook enables the application to quickly respond to the results. This model can be incorporated into a CRM system, document workflow, or any application requiring programmatic fax support.