SF doesn't fax. But it will email, and there are 3rd party email2fax services out there (efax, etc.).

But the 3rd party email2fax services don't work with SF email headers. They can't verify that the email is coming from one of their paid customers, so they reject the email.

Does anyone know email technicalities well enough to make an educated guess as to whether implementing SF Email Relay would resolve this issue? If I use SF Email Relay, would my SF emails generate email headers that look exactly like the headers I get if I send an email through my email client?