Email spoofing

You can certainly change all of the header fields which originate with your computer. You cannot, however, spoof any fields which are added once the mail has been sent. The IP address of the computer/network from which the mail was sent, for example, cannot be faked (though it may be a proxy address, very few spammers bother to use one) and it is possible to identify the machine and ISP used to send the mail. When reporting spam, the ISP supplying the Internet connection is legally responsible for spam and other illegal activities and, once one has identified the ISP, one should send a copy of the mail, including the full header to abuse@[ISP's domain]. They should take such reports seriously and investigate them, though it has to be said that one is very unlikely to get a response from ISPs in certain countries, which are very often the very places that spam is sent from!