OS X 10.9.2, Server.app 3.0.3
So, Apple wants users to redirect (not forward!) mail to junkmail@ and notjunkmail@ to retrain the spam filter. I think that sucks --- this is not a reasonable way to do things in my opinion.
Anyway, the mechanism seems to be via the launch daemon configured at
/Applications/Server.app/Contents/ServerRoot/System/Library/LaunchDaemons/com.apple.salearn.plist