The MAC addressed is assigned to a computer connecting to the ISP, but probably not logged by brand/serial #. Take your laptop to the local McDonalds that has WiFi and it will be assigned a different MAC address than when you use it on the internet at home since most ISP's use dynamic assignment of MAC addresses to conserve bandwidth. Heck, for most ISP's these days a WiFi modem/router is standard and it's the modem/router that is assigned the MAC address, not the individual computers. The router, in turn, assigns addresses to the individual computers connected to the LAN (your home wireless/wired network). So you could have 2 desktops, a laptop, an IPad, and an internet connected TV all connected to the internet through one MAC address as far as the ISP is concerned.
That's not to say that someone couldn't take the ISP's logs plus the header info from an email and track down a specific computer, especially if they had access to the suspect computer, but it wouldn't be as simple as you suggest and I suspect in most cases would require a court order to get the ISP's logs and the suspect computer. Of course, if the suspect computer has been "stolen" it becomes more difficult...
Jim