Are you sure you're just not asking for the confimation mail to be sent to the same address each time?
There's no way for the remote host to know the MAC from beyond routers unless they're sending a control of some type as well to be executed by the client. Within a single network (that will accept broadcast messages) it's another story.
http://www.codeproject.com/internet/winarpspoof.asp
All on spoofing (changing the MAC):
http://en.wikipedia.org/wiki/MAC_address They did not mention that some network cards allow direct manipulation of the "burned-in address" on the card hardware itself.