Short answer
No, and no service can encrypt a fax end to end: the T.30 protocol makes no provision for it, and the last leg to the fax machine travels over the telephone network in the clear. MondialFax encrypts the path between your browser and its servers, then deletes the document once the fax has gone.
Why a fax cannot be end-to-end encrypted
A fax is transmitted under the ITU-T T.30 standard, published in 1980: two devices talk to each other in tones over a telephone line. The standard provides no encryption, and the recipient's fax machine would not be able to decrypt a page that arrived encrypted. No fax service, free or paid, can therefore encrypt a fax all the way to the device that prints it.
What MondialFax encrypts
The path between your browser and the MondialFax servers is encrypted (HTTPS): nobody can read your document while it is being uploaded. On the servers, it is used only for the transmission and is deleted once the fax has gone.
The telephone leg, from the servers to the recipient's fax machine, travels in the clear, as with any fax.
The real risk, and sensible precautions
Tapping a telephone line takes resources few people have. The everyday risk lies elsewhere: a mistyped number, or a page coming out on a shared fax machine in a corridor or a front office. Check the number twice, let the recipient know the fax is coming, and write “confidential” and the recipient's name on the cover sheet.
For a highly sensitive document, consider whether an encrypted channel the recipient accepts — a secure messaging service, an upload to a personal online account — might be a better choice.
Key points
- No fax is end-to-end encrypted: the T.30 protocol does not provide for it.
- Uploading to MondialFax is encrypted (HTTPS).
- The document is deleted after sending.
- The main real-world risks: the wrong number and the shared fax machine.