I give a thumbs up to Declude (it can be resource heavy, using all the resources on your Mail Server that it can, but it is definitely effective).
We also setup a Custom Rule in Smartermail ANTISPAM ADMINISTRATION as follows:
Rule Source: Header
Rule Type: Regular Expression
Rule Text: .+\.us>$
Notice that if you are using 10/20/30 for Low/Med/High probability it isn't enough on it's own to flag an email as Low Probability as there are many legitimate domains with the .us extension (especially local & state government departments). However, it is generally enough to tip the scales if they fail at least one RBL, URIBL, Spam Assassin, or Bayesian Filtering.
We do similar for other domains, such as .CLICK, .CRICKET, .DATE, .DOWNLOAD, .LINK, .MEN, .PARTY, .REVIEW, .ROCKS, .SCIENCE, .SPACE, .STREAM, .TOP, .WIN, .WORK, .XYZ, .ZIP but with a 20 Weight as I have yet to see a single legitimate piece of email come from any of these domains.