We can fax! - Fax2Mail and Email2Fax with T.38 at dus.net

Many organizations continue to work with fax - for example where written form, archives or interfaces to older systems play a role. At DUStel, we bundle fax with the VoIP infrastructure of dus.net: Incoming faxes can be delivered as Fax to eMail as PDF (with optional original TIFF attached); outgoing faxes can be sent using Email2Fax or eMail to Fax with PDF attachment; in parallel, T.38 supports the operation of an existing fax device via a suitable adapter such as a Fritz!Box.

The three sections below explain Fax to eMail, T.38 and eMail to Fax individually - including the stated fee of EUR 0.03 per received Fax2eMail page (incl. VAT) and the note that a free Fax2eMail service is no longer offered. Sending from the customer center and sending by e-mail address rufnummer@fax.dus.net with a subject line for authentication correspond to the processes described there.

Incoming faxes to your telephone number are converted directly into a PDF document on our system and sent to you by e-mail.

The PDF is well suited for forwarding, filing in document management or printing on demand; the optional TIFF corresponds to the classic fax format and can be helpful for post-processing or special tools.

Our fee-based Fax2eMail service (EUR 0.03 per page received, incl. VAT) is based on highly efficient and extremely reliable software that provides you with the original TIFF in the e-mail attachment in addition to the PDF document. A free Fax2eMail service is no longer available.

With the T.38 protocol, we create a reliable way to connect the old world with the new. Your existing fax machine remains your primary input and output device.

In addition to your fax machine, a T.38 "adapter" is required for operation. The Fritz!Box, for example, is such an "adapter" that supports the T.38 protocol and thus makes your fax machine T.38-capable.

Mixed operation in conjunction with fax to e-mail is also possible - for example, if some of the correspondence should end up on the device and another part directly as a PDF in the mailbox.

In addition to sending a fax (for a fee) directly from the customer center, you can also send a fax by e-mail with an attached PDF. This is easy, convenient and possible from anywhere.

The recipient fax number is entered as the email destination address (rufnummer@fax.dus.net) and the document to be faxed is attached as a PDF. The subject line is used for authentication.

You will then receive a PDF dispatch report by e-mail, with a thumbnail of the 1st page and the dispatch parameters as well as the dispatch status.

You don't need a fax machine at work: the PDF report in the e-mail summarizes the status and dispatch parameters - easy to read for you or for queries to support.

Fax machine and classic office communication

Legally compliant fax with proof of delivery

Your fax. Your proof.

Our delivery report serves as the functional equivalent of a courier delivery: as an independent third party, dus.net confirms not only the successful delivery but also the complete content of the document sent. In Germany, fax is still considered a recognized method of transmission with high evidentiary value—we turn it into a verifiable proof of delivery.

SHA256 fingerprint of the original

Before each conversion, we calculate the checksum of the PDF file you submitted and include it in the submission report—even if the upload fails—as proof of submission.

Anyone who later submits a file with an identical fingerprint proves that this exact document was sent.

This feature can be enabled in the Customer Center (Phone System → Send Fax → Advanced Settings); the fingerprint can also be viewed at any time in the fax call detail report.

Here's how to check that

sha256sum ihr-dokument.pdf

Broadcast report with all pages

If requested, the transmission report will include all pages of the faxed document in full size, rather than just a thumbnail of the first page.

No one can claim that pages 2 through 12 were blank—the complete contents are part of the record.

Up to 25 pages. Can be enabled as a customer setting or on a per-mail basis by adding “:fullreport” to the subject line.

Here's how to check that

Open the transmission report — every faxed page is included in its original size, not just the first one.

Digitally signed transmission report

Each report is cryptographically signed by dus.net (PKCS#7/CMS signature attached as fax.pdf.p7s).

Anyone can verify the origin and integrity of the data using standard tools such as OpenSSL.

The test certificate is available for download in the Customer Center.

Here's how to check that

openssl cms -verify -in fax.pdf.p7s -inform DER -content fax.pdf -CAfile dusnet-cert.pem

GPG-encrypted delivery

If a public PGP key for the recipient's email address is stored on the key servers (keys.openpgp.org), delivery reports and fax notifications are automatically sent using PGP/MIME encryption—without any configuration.

The key lookup is performed via keys.openpgp.org; encryption is applied if a matching key is found there.

Here's how to check that

gpg --decrypt sendebericht.eml

When proof is what matters

Reminders, notices of termination, time-sensitive matters, and communications with government agencies require more than just “sending.” Law firms, property management companies, insurance companies, and SMEs use every fax to comprehensively document what was sent to whom—the fingerprint, full page content, and signature together provide proof that stands up to later scrutiny. Fax instead of certified mail, without a trip to the post office.

Frequently Asked Questions About Legally Valid Faxes

The fingerprint is a unique checksum of the PDF file you submitted. If you later submit a file with the same fingerprint, this proves that it is, down to the last bit, the exact same document you sent. Even a single changed line would result in a completely different value.

Each transmission report includes a PKCS#7/CMS signature as an attachment fax.pdf.p7s. You can use OpenSSL and the verification certificate from the Customer Center to verify the origin and integrity of the report:

openssl cms -verify -in fax.pdf.p7s -inform DER -content fax.pdf -CAfile dusnet-cert.pem

No. This is a CMS/PKCS#7 signature using a dus.net certificate that verifies the origin and integrity of the transmission report—not a qualified electronic signature under eIDAS. It is sufficient to verify who created the report and its integrity, and can be verified by anyone using standard tools.

Enable the full report as a customer setting in the Customer Center or for each individual transmission using the subject line suffix :fullreport. The report will then include all faxed pages in full size—up to 25 pages—instead of just a thumbnail of the first page.

As soon as a public PGP key for the recipient's email address is stored on the key servers (keys.openpgp.org), delivery reports and fax notifications are automatically sent using PGP/MIME encryption—without any further configuration. If no key is found, delivery proceeds as usual.

The verification features—fingerprint, full report, signature, and GPG encryption—are included in the fax service at no extra charge. Standard fax rates apply: EUR 0.03 per Fax2eMail page received (including VAT); outgoing faxes are billed according to the Connection Price List.

Faxes with legal validity — use them right away

Existing customers can enable the tracking features in the Customer Center. New customers can find the right fax plan in the plan overview.

Activate in the Customer Center View fax rates