The MAC address does not survive beyond IP routers. You can't find the client MAC address from a remote server.
In a local subnet, the MAC addresses are mapped to IP addresses through the ARP system. You can find the MAC address of computers within your subnet using 'arp -a' or 'nbtstat -A ipaddress' command in windows. However, when your packets passes though the router from the local subnet through the gateway out to the Internet, the originating MAC address is lost.