Try out the Preferences interface by turning off the folder cache. This display just takes up space if you don't have many folders. If you have lots of nested folders, though, you might even want to make this display bigger!
Click on the Preferences button, which brings up a dialog with buttons for several of the modules that make up exmh. Click on the Folder Cache button to bring up the preference items that control the folder cache. In this case there are just two items: the number of lines of labels in the cache and the names of folders always in the cache. Click in the first field and backspace over the default value of 1. Type in 0 instead, and press <Return>. Voilà! The folder cache disappears.
If you like this setting, click Save in the main Preferences dialog and exmh saves your changes in a file named ~/.exmh-defaults (the tilde, ~, stands for your home directory). Click Reset if you want to undo your changes. Be a little careful here because you are allowed to Dismiss the Preferences dialog without saving.
Another useful preference item to set is under Background Processing. You can arrange for exmh to run inc periodically and automatically transfer your messages into your inbox. The advantage of doing this is that the folder label highlighting works best this way. Unfortunately, exmh does not give you any visual clues when mail is only waiting in your system mailbox.
The Section Preferences gives more details about the Preferences dialog.