Email spoofing refers to email that appears to have been originated from one source when it was actually sent from another source. Individuals, who are sending "junk" email or "SPAM", typically want the email to appear to be from an email address that may not exist. This way the email cannot be traced back to the originator.
There are many possible reasons why people send out emails spoofing the return address: sometimes it is simply to cause confusion, but more often it is to discredit the person whose email address has been spoofed: using their name to send a vile or insulting message.
Sometimes email spoofing is used for what is known as "social engineering", which aims to trick the recipient into revealing passwords or other information. For example, you get an email from what appears to be the LSE's email administrator, or from your ISP, asking you to go to a Web page and enter your password, or change it to one of their choosing. Alternatively, you might receive an email asking for detailed information about a project. The From field suggests that the message comes from the LSE, but instead it is from a competitor.
Dealing with a Spoofed Email
There is really no way to prevent receiving a spoofed email. If you get a message that is outrageously insulting, asks for something highly confidential, or just plain doesn't make any sense, then you may want to find out if it is really from the person it says it's from. You can look at the Internet Headers information to see where the email actually originated.
Remember that although your email address may have been spoofed this does not mean that the spoofer has gained access to your mailbox.