This is an extremely weak method of authentication, MAC addresses can be spoofed easily. You can usually notice the spoof if you've got your network monitoring set up correctly, but that depends on network topology as well. Also, if the data is confidential you shouldn't be sending it across an untrusted network unencrypted. I would suggest using ssh for this (since the recipient machine is a Linux box which has an ssh daemon already installed), that gives you host authentication for free with the encrypted channel.