If such a list existed, every cretin would harvest it and you would be buried in junk. Get ready for this. Most people send jokes to more than one person at a time. Usually, when someone receives the joke, they simply forward it to their friends, and so forth, and so on. After a while, these jokes have so many addresses on them, you have to scroll quite a bit just to see the funny story.
Many spammers have software that can extract e-mail addresses out of messages.
Even if you don't forward jokes to others, if anyone has sent a joke TO YOU, your address becomes public knowledge. They will all still receive the joke, but their addresses will not show up on the messages. Not only are you protecting their privacy, you are preventing spammers from using their addresses. In other words, if your user name is a dictionary word, you could get spam automatically. Common user names are also thrown into the mix.