To recieve fax messages and send them via email:

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

And done.