You don't want your mail to go to the inbox? What? Why would you want that?
But OK, if you truly want to send an email message and ensure that it goes to the spam folder, The Next Web reports on a tool called Straight2Spam, for those times when you want to email somebody but want them to possibly miss the email by having it delivered to the spam folder. It sounds like a fine opportunity to engage in passive aggressive behavior, if you ask me.
You might be wondering, how well does it work? I have no idea. I've got a full time job and no spare cycles to test this kind of nonsense myself. You should try it out and let me know if it gets the job done.