Also the limitations of fax sort of end up being it's differentiator to email and it's biggest advantage. Not needing an email server is a big boon, not really being susceptible to phishing is a boon, and with modern fax over internet it's virtually indistinguishable in user experience from email.

I remember fax phishing even before I had ever heard of email. From many large companies, simply paying a sub $100 invoice was standard procedure without even checking with the other internal bodies.

This is true, but it's much less of a concern because:

1. You get way less faxes than emails.

2. Faxes can't steal credentials.

3. You should be auditing expenses anyway.

If only a standard existed to do this... Hint: it exists since ages in Italy and it has been extended to Europe recently (See Registered Electronic Mail - RFC 6109 and ETSI EN 319 532 – 4)