> It is more secure as long as you regard PSTN as more secure than internet. Which is a view that has some merit.

Not as long as you can send faxes from the Internet.

https://faxzero.com/

FaxZero even allows you to put in the phone number the fax comes from. It's $1.99/fax to remove the FaxZero cover letter, and they accept PayPal.

(Also, someone with the username iptel might know a bit about this.)

I meant it in the confidentiality sense. The fact that you can put anything on the cover sheet and header line as sender number and that it has exactly zero relevance as authenticator should be obvious to anyone who actually ever set up a fax machine.

Edit: and exactly how that differs from SMTP?

An office fax machine sitting out in the open is going to be less confidential than email on an SMTP server where only a sysadmin or the recipient can see it, yes.