email fax security

I am considering signing up for one of the email fax systems such as
MyFax.

http://www.myfax.com/

In reading over the FAQs, I learned that MyFax uses the destination
phone number as the email address. If I want to send a fax to
888-555-1212, I send an email to .

If it's just a short message, I can type it in the email message. If
it's a document, such as a Word document, I attach the document to the
email.

I am wondering how secure this is? Some of my documents are
confidential and some of them are legally privileged. When they leave
my machine and go out over the internet, those attachments are not
encrypted, right? They will pass through a number of nodes enroute to
myfax.com. Could some at one of the intermediate nodes look at the
attachments?

Thanks