Bluehost recently switched over from POP3 to IMAP.
After scratching my head over why I couldn't access my email from Horde, I gave tech support a call.
It was rather amusing to give my email's password over the phone -- "capital sierra, papa, alpha, mike, capital mike, echo, capital romeo, sierra, dollar sign, sierra, uniform, charlie, capital Kilo, tilde, alpha, capital sierra, sierra". While "SpamMeRs$sucK~aSs" is not my real email password, you get the idea. Anywho, I think I convinced the tech support guy that I wasn't a "Where's the any key" type user.
Whilst on hold, I figured out what the deal was -- my custom procmail setup was dumping stuff into an mbox format, rather than an IMAP folder. Since everything was now IMAP, Horde was having issues.
So, after about 17 minutes with the tech support guy (thank [insert supreme being of your choice here] for toll-free tech support), they converted my old mboxes to a sexier IMAP format.
After a couple of test emails, I had my procmail sorted as well. Basically, you need to change the target from an mbox to an IMAP folder. QED.
This is just another reason I will recommend Bluehost to anyone who asks. While I've had some minor hiccups in the past, they have been seriously on the ball with their tech support.