Fax machines can provide a unique service. Anything manually faxed from one fax machine to another one is secure. It's not hackable in the way e-mail is. Since it's sent over a phone line there is no record of the file on any computer, server or PC. Not anywhere.
The FBI could probably tap your line and capture any faxed documents, but that’s highly unlikely unless you are a major criminal.
Online faxing has the issue of everything being sent to an e-mail account. Then, it's being hosted on an e-mail server and a personal computer. So, it could be compromised in the same way that any e-mail account could be. Thus, the original machine to machine technology looks like the most secure technology today. It's still used in various secure environments today for that very reason. Title companies have manual fax machines used for sending and receiving banking info.
The way faxes are secured in the digital world is thru a ‘secure portal’ access. In the medical field, there is a regulation for HIPAA Faxing online. The fax is only accessible through a password-protected portal. You get a notice that you received a fax and then you have to log into a secure portal to retrieve the fax. That helps protect against an unauthorized user, or employee. However, common practice is that once the fax is retrieved the user saves it on their local computer. If the computer gets compromised the fax is there for anyone to see. So, machine to machine faxing is really the most reliable way to securely send a document.
Here's our suggestions on what to do with your old fax machine.