Fighting spam is a little bit like fighting computer viruses. It is a constant battle between the Spam detection programs like Spam Sleuth and spammers. We know what some of the spam looks like because we've seen it before, but unfortunately, there will be new things to sell and unscrupulous companies out there that will try to hawk their wares using spam.
What makes an e-mail spam? Technically, spam is an e-mail that you didn't request, that is commercial in nature and is trying to sell you something or get you to do something. If you signed up for a newsletter, and that newsletter has a sales pitch for the company's product, it isn't technically spam. If you forgot that you signed up, then it sure seems like spam when it arrives.
How does Spam Sleuth distinguish between spam and a legitimate newsletter? That is very hard to do, because the spammers try to convince you that you did sign up with them, or their "marketing partner." Since there isn't currently a way to distinguish the two, we simply define spam as e-mail that you don't want.