Spoofed e-mail appearing to be legitimate. Click on image above to enlarge.
In the context of network security, a spoofing attack is a situation in which one person or program successfully masquerades as another by falsifying data and thereby gaining an illegitimate advantage.
The sender information shown in e-mails (the "From" field) can be spoofed easily. This technique is commonly used by spammers to hide the origin of their e-mails.
E-mail address spoofing is done in quite the same way as writing a forged return address using snail mail. As long as the letter fits the protocol, (i.e. stamp, postal code) the SMTP protocol will send the message. It can be done using a mail server with telnet.