CTS often has users contacting us that they receive email, apparently from them, but contains spam. This is a common spammer tactic called "e-mail spoofing" and is an attempt to use a legitimate e-mail address to fool anti-spam measures. Unfortunately, this cannot be prevented with current technology.
A detailed explanation of E-mail Spoofing can be found here: http://en.wikipedia.org/wiki/Email_spoofing
An excerpt from the article:
"E-mail spoofing is a term used to describe fraudulent e-mail activity in which the sender address and other parts of the e-mail header are altered to appear as though the e-mail originated from a different source. E-mail spoofing is a technique commonly used for spam e-mail and phishing to hide the origin of an e-mail message. By changing certain properties of the e-mail, such as the From, Return-Path and Reply-To fields (which can be found in the message header), ill-intentioned users can make the e-mail appear to be from someone other than the actual sender."