The email to fax gateway is a work in progress. Here a small explanation of the access control mecanism it proposes. This may evolved in the future. Comments are welcome.
An access rule may be open or selective. Mostly, you can specify that faxes are accepted from anywhere, only from the local net, only from fax users or only from the member of the user list of this rule.
You may required that email from fax users be PGP signed so you are sure they come from this user.