You can't send files that way. If you send something to the fax mailbox, it must have [faxphone=####] in the subject. That number will be transformed to an email address fax=####@gateway.com, where gateway.com is the suffix for the router. If gateway.com doesn't exist in DNS, then the fax service will--after rendering the file--send you an NDR back with the rendered file. So if you just want to test the rendering without a gateway, that might be an option.