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Enterprise Fax APIs by Retarus – for Developers Who Want More Than Just an Interface

Thanks to Retarus Fax APIs, Sending and Receiving Faxes with Your Application Just Got Super Smart.

Whether mobile apps or web applications—find the right interface for your development environment with the Retarus fax APIs. And best of all, they offer all the key things you need to successfully use Cloud Fax: a powerful infrastructure, international availability, and numerous customization options to fit your business operations.

Retarus Cloud Fax APIs: Developer Friendly and Intuitive

Send and receive faxes with your web or mobile application now. Simple and reliable with the Retarus Fax API. Via REST API, SOAP API or alternatively via FTP or SMTP. It’s quite easy and developer intuitive. A few lines of code and you’re ready to go with our fax API:

curl -i 
  -H "Authorization: Basic [InsertYourCredentialsHere]" 
  -H "Content-Type: application/json" 
	-d '{
  "recipients": [
      "number": "+49891234567890"
  "documents": [
      "name": "example.txt",
      "data": "SGVsbG8sIHRoaXMgaXMgYSB0ZXN0aW5nIGRvY3VtZW50IGJvZHk="
  "renderingOptions": {
    "paperFormat": "A4"
  "statusReportOptions": {
    "reportPurgeTs": "2018-08-07T13:34:37.098+02:00"
echo '
<soapenv:Envelope xmlns:soapenv="" xmlns:ns="">
' > request.xml
curl -i 
  -H 'Content-Type: text/xml;charset=UTF-8' 
  -H 'SOAPAction: ""' 
  --data @request.xml

Reports can be sent via email or through an http web service. For additional sending options you can refer to the following WSDL:

<?xml version="1.0" encoding="iso-8859-1"?>
<!DOCTYPE jobs SYSTEM "job.dtd">

To send a fax using FTP you just need to load on the Retarus FTP server the fax document (e.g. minimal.pdf) and an XML file in the format. The XML file supports most of the options available in other APIs for personalizing each sending. Reports can be retrieved through the same FTP interface.

Through Retarus Email-to-Fax it is possible to send faxes manually through any email client, or in batch by connecting your application through SMTP to the Retarus infrastructure.


Alternatively, a distribution list can be defined as a text file and the message will be sent to all recipients in the list. In this case the destination email address is:


A report will be returned to the sending email address. Further options can be specified by inserting tags in the subject line, for example:

<code>[PRIO:URGENT]</code>: requires an express transmission
<code>[ATTONLY:ON]</code>: only the attachments, not the body of the email, will be sent as fax
<code>[RES:HIGH]</code>: The fax will be sent in high resolution.
Call Fax Transmission With “All Options”:
curl -i 
   -H "Authorization: Basic [InsertYourTokenHere]" 
   -H "Content-Type: application/json" 
   -X POST 
   -d ' {"reference":{"customerDefinedId":"myCustomerDefinedId","billingCode":"myBillingCode","billingInfo":"myBillingInfo"},"recipients":[{"number":"004989312000000000","alternativeNumbers":["004989312000000000123"],"properties":[{"key":"key123","value":"value123"}]},{"number":"0049893120000000002","alternativeNumbers":["004989312000000000123"],"properties":[{"key":"key123","value":"value123"}]}],"documents":[{"name":"document.suffix","charset":"UTF-8","reference":null,"data":"SGVsbG8sIHRoaXMgaXMgYSB0ZXN0aW5nIGRvY3VtZW50IGJvZHkgY3JlYXRlZCBmb3IgOTk5OTlURQ=="}],"transportOptions":{"csid":"csid-inserthere","isExpress":true,"isBlacklistEnabled":false},"renderingOptions":{"paperFormat":"A4","resolution":"HIGH","coverpageTemplate":"coverpage-default.ftl.html","overlay":{"name":"retarus1p","mode":"ALL_PAGES"},"header":"%tz=CEST Testfax: CSID: %C Receiver Number: %# Date: %d.%m.%Y %H:%M %z"},"statusReportOptions":{"reportPurgeTs":"2018-08-07T13:34:37.098+02:00","reportMail":{"successAddress":"","failureAddress":""},"httpStatusPush":{"targetUrl":"","principal":"examplePrincipal","credentials":"examplePassword","authMethod":"HTTP_BASIC"}},"meta":{"customerReference":"XXXXXXX","jobValid":{"start":"2016-08-14T14:02:01.497+02:00","end":"2016-08-14T15:02:01.497+02:00"}}} '
Call Fax Receipt via API:
curl -X POST -u T1:
Product Documentation Center

Interested in More APIs and Details?

In our Documentation Center you’ll find a collection of API reference documentation, guides for integrating our products into your business as well as SDKs for popular web languages, code snippets, tutorials, and sample apps for common use cases.

Visit the Retarus Product Documentation Center

Graphic: Developer Portal for SMS, Email and Fax API

Discover the Benefits of Retarus Cloud Fax.


Excellent transmission rate

Reduce your fax transmission failure rate by up to 10% and reduce the administrative costs involved in manually processing your faxes. Retarus' NeverBusy Technology ensures that multiple faxes are never sent to the same recipient simultaneously. Thanks to this technology, even high volumes of incoming faxes are delivered immediately—no busy lines or capacity bottlenecks.


Intelligent routing

Achieve up to 30% more throughput with Cloud Fax. Thanks to Active Carrier Management, Retarus automatically uses the lines and carriers whose line quality and availability best match your fax job.

Scalable and flexible

Eliminate over- and under-utilization of capacity with Retarus Cloud Fax and our fax APIs. Regardless of whether you send 1,000 faxes today and one million tomorrow—Retarus provides you with the exact bandwidth you need at any given point in time without a single fax server in your office, all thanks to our fax APIs.

Highly available and global

Rely on reliable transmission worldwide. Retarus offers high availability and high capacity, with its own data centers in Europe, Asia, and the USA, providing maximum availability and delivery reliability. 24/7.


OCR and barcode recognition

Retarus Cloud Fax enables you to seamlessly integrate faxing into your workflows. Read out information or barcodes from incoming documents with the OCR and barcode recognition feature and then use customized smart routing options to presort, forward, store, or process the documents automatically according to your rules.


Perfect integration thanks to Fax APIs or standard protocols

Whether you use an ERP, a CRM or a legacy system, Epic, Microsoft 365, or Google Workspace — thanks to open standards, you can connect Retarus Cloud Fax to almost any business or cloud application. For example, via RESTful-API web service or via standard interfaces and protocols such as SMTP, SFTP, and HTTPS.

Even More Benefits for You …


Monitoring and analysis

Check the status of your fax communications at any time using dashboards and detailed reports in the web-based Enterprise Administration Services Portal (EAS). In real time or in user-defined periods. For both outgoing and incoming faxes.


Completely transparent costs

Reduce your investment and process costs. Retarus lets you massively reduce or completely eliminate costs for telephony and fax infrastructures, software, updates, administration, and maintenance. Retarus' billing models are based exclusively on usage-related batch and page prices (pay-per-use).


Security and compliance

Rely on maximum security. With its global data centers, Retarus fulfills the most stringent country- and industry-specific requirements for data protection and data security including ISAE 3402 Type II and PCI DSS certifications, HIPAA and GDPR compliance, and many more


PGP and X.509 encryption

Ensure highly secure online faxing communications. Connect to the Retarus Messaging Platform exclusively via encrypted connections (TLS, VPN). Cloud Fax will also sign and encrypt incoming fax documents for you upon request—depending on the file type per AES 256-bit, PGP, or X.509.


Long-term archiving

Ensure full compliance with industry-specific or statutory regulations. Retarus' long-term archiving helps your fax communications comply with the most stringent data security and integrity requirements.


Enterprise-level SLAs

Choose a service level agreement that meets your needs in terms of quality, delivery speed, uptime and response time. Retarus provides flexible and customizable SLAs to help fulfill your unique business processes. Retarus fax APIs are suitable for startups, mid-sized companies, and international enterprises.

Fax Number Provisioning

Receive faxes anywhere in the world with local numbers. Retarus provides service and fax numbers on all continents that can be managed and assigned easily in the EAS Portal. It is also possible to port existing incoming fax numbers at any time. Want to continue to own your fax numbers but use Retarus Cloud Fax? Retarus can manage various SIP integration scenarios to match your needs.

Service and support

Experience first-class support. Our service and support team is here to answer your questions, tailor a solution to your needs and provide cloud fax that exceeds your expectations.

Retarus Enterprise Fax APIs: Try for Yourself!

The Retarus Enterprise Cloud is a unique global infrastructure for reliable information transfer via fax, email, and SMS. Since 1992, Retarus has been on the forefront of providing advanced fax communications. Incorporate our expertise in faxing in your processes today. Easy, fast, and reliable.