To recieve fax messages and send them via email:
in Settings → Fax Options fill in email addresses, other fields can be left with defaults
in Extensions → Add new Generic SIP Device extension with extension name and number
Edit created extension and in Fax section enable fax and fill in destination email address
in Connectivity → Inbound routes create new route for that extension:
Detect Faxes to Yes
Fax Detection type: SIP
Fax Detection Time: 8
Fax Destination: Fax Recipient → Created extension
And done.