I had to do that when we were testing out some major tweaks to Spam Assassin. I ended up getting a throw away domain and making sure emails got plastered every where they possibly could.
Then, low and behold .. and endless supply of 100% USDA Grade A SPAM was delivered to our mail server.
The funny thing is, we shelved (parked) the domain for almost a year, found a need for it again .. the minute we pointed it at our mail servers the SPAM started again.
Now I use it to predict lottery numbers (though, it has yet to work). I gather entropy based on SPAM e-mails sent from the future :)