A Joe-job is spam that uses a forged return address with the specific intention of damaging or tarnishing the reputation of the forged domain.
The term comes from such an action used against Joe's Cybershop in 1996. Joe Doll (proprietor) provided free web pages under certain conditions. One 'customer' advertised his page using spam. Joe Doll warned him to stop. The customer stopped then restarted. Joe Doll removed the customer's web page. “The response was swift, massive and ugly. It included threats, forged messages to spam lists, and mail bombs. Enraged victims have mounted mail, ping, syn, and other attacks on joes.com, incited to vigilante justice by the forger,” Doll subsequently wrote.
The spammer had retaliated by further spams, but this time using Joe Doll's address as a forged From address.
A Joe-job has two basic effects. Innocent spam victims can be duped into thinking that the spammer is the forged address. These victims can respond angrily either directly to the joe, or indirectly to his ISP - where the danger is that the joe could even lose his website. In reality, most people can recognize a Joe-job when they see one - which just leaves the second effect: the joe's website can be overwhelmed by hundreds of thousands of bounced spam messages bouncing back to the From address.
Strictly speaking, since the term is based on the Joe's Cybershop incident, it should apply solely to address forgeries where the motive is to do harm to the joe. Without this motive, it is just another example of email-spoofing spam. It is likely, however, that the term will come to be used to describe the effect of any large scale spam that uses a forged From address.