First off, thanks so much for this! Our faculty have been begging for a course-based email system for Moodle, and this seems to fit the bill perfectly.
One issue I've encountered in testing relates to Labels. We can create a label no problem, and then we can move a message to the label. The problem is when we click on the label name, nothing happens, we stay on the message list for the inbox. If I click on draft, sent mail, or Bin, it's the same thing, the messages in those "folders" don't display. If I search for the messages individually, I can find them, but if I click on Inbox again, it does not update the display. I have to click "Check" to display the inbox again.
I've tried this with IE, FF and Chrome and get the same behaviour with all browsers.