Spams that mimic real emails

I always felt the spam emails I receive mimic the real emails in my inbox. I spoke to our web admin about this several months ago since I was worried that our mail server was infected by some viruses that "learn" the patterns of our real emails. Today, it just occured to me: it is NOT that the spams mimic our real emails. Rather, ONLY spams that look close to our real emails get pass our spam filter. I don't know why I didn't think of this earlier.