A while ago I wrote an article called Training SpamAssassin, Horde, IMAP, Plesk user configuration which describes how to integrate Spam Assassin user learning and Plesk's installation of Horde mail client.
Sam left a comment suggesting that I tell Horde about the good work I have done - so I did!
Opened an enhancement bug request and after a bit of a chat they agreed to put my changes into IMP 4.2. Hurray!
I'll wait for 4.2 release then update my original article.