Types of E-mail spoofing

Man-in-the-middle
In this form of network attack, a hacker will intercept two parties communications, and then alter the communication in any way that he/she sees fit. By using this form of spoof, a hacker can convince the receiver of a message to disclose confidential information, since the message will appear to have come from the supposedly trusted third party (the original sender of the message).

Non-blind spoofing
This occurs when a hacker is using the same subnet. The sequence and acknowledgment numbers are changed which makes it hard to calculate correctly. The largest problem with this type of spoofing is session hijacking, allowing a hacker to bypass any security set it place within the connection.

Blind spoofing
Blind spoofing is a much more difficult attack because the sequence and acknowledgment numbers are not reachable, making them extremely hard to track down and change. This is overcome by sending packets to the system being attacked to provide a sequence of numbers to discover the formula by studying these packets. Once the formula has been discovered, the sequence and acknowledgment numbers can be changed allowing the hacker full access.

http://en.wikipedia.org/wiki/E-mail_spoofing