We were helping a good friend of ours with an email campaign, and during testing, we noticed that the emails were getting spam filtered by Yahoo!Mail. Weird. The subject line was honest and to the point, the colors were pretty much all web-safe, the coding was fine, and we didn’t have any "pushy" sales copy (like "BUY NOW!") in the message. After tinkering a bit, we noticed our placeholder text in the right column was the culprit (where it says, "tis the season!" 3 times). All we had to do was remove the exclamation points at the end of each sentence, and voila—Yahoo let it through. Goes to show how sensitive and weird those spam filters can get.
And you might find this article handy for understanding how spam filters think.