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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 fax services: 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 Mail2Fax 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:
Retarus Developer Portal

Interested in More APIs and Details?

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

Visit the Retarus Developer Portal.

Graphic: Developer Portal for SMS, Email and Fax API

Discover the Benefits of Retarus Enterprise Fax APIs.


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 our cloud fax API. Active Carrier Management in the Retarus Enterprise Cloud 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 our online fax services and fax API. Regardless of whether you send 1,000 faxes today and one million tomorrow—the Retarus Enterprise Cloud provides you with the exact bandwidth you need at any given point in time without a single fax machine in your office, all thanks to our fax APIs.

Highly available and global

Rely on reliable transmission worldwide. The Retarus Enterprise Cloud 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' enterprise fax services enable you to seamlessly integrate faxing into your workflows. Read out information or barcodes from incoming documents with the Retarus 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 Services 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 Retarus Enterprise Administration Services Portal. 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 telephone 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. The Retarus Enterprise Cloud, with its global data centers, fulfills the most stringent country and industry-specific requirements for data protection and data security including ISAE 3402 type II and PCI DSS certifications, GDPR and HIPAA compliance, and many more.


PGP and X.509 encryption

Ensure highly secure online faxing communications. Connection to the Retarus Enterprise Cloud is exclusively via encrypted connections (TLS, VPN). The Retarus fax services 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 and response time. Retarus fax APIs are suitable for startups, mid-sized companies, and international enterprises.

Telephone number service

Receive faxes anywhere in the world with local numbers. Retarus provides service and call 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.


Service and support

Experience first-class support. The Retarus Team for Enterprise Fax APIs is here to answer your questions, tailor a solution to meet your needs and provide a secure fax service.

Retarus Enterprise Fax APIs: Try for Yourself!

The Retarus Enterprise Cloud is a unique global infrastructure for reliable information transfer via fax, email, text message, and EDI. Since 1992, Retarus has stood for technically advanced fax communications for businesses. Incorporate our expertise in fax transmission in your software today. Easy. Fast. And reliable.