Email forging has always been one of the easiest forms of email hacking ever used. It has been well known in the hacking community for quite a long time.
All one needs to know while forging email, is the basic knowledge of how the SMTP works. SMTP stands for Simple Mail Transfer Protocol - The protocol(set of instructions and rules) used to send email in the Internetwork. The total set of the step by step instructions from client to server can easily be found in any rfc database. Click here for the rfc relating to the SMTP. This link is for those, who are real PUJARIES of ultimate knowledge. Not for dummies...
A tutorial on how to forge email through any command prompt, can be very easily found in the Internet. So, I will not reinvent the wheel by redescribing the whole process here. The following link will show the simple tutorial on how to forge a simple email. Advanced email forging including attaching files in the mail will be posted soon in this blog. Keep checking.