Occasionally customers run into issues trying to configure this app given that there are huge myriad of email servers used and many of these have protocol nuances that see to cause issues. Here are a couple strategies to get things working:
Try configuring the app against a Google Mail account
This can be a Google Apps mail account or even a personal mail account. Doing this verifies that the app is itself indeed working. If needed I can even provide such an account though its easiest just to create a temp account oneself.
Forward to a Google Mail account
If the app is having trouble interacting with your email system, consider setting up a Google Mail account (via Google Apps or a personal type account). Then simply forward your emails to that account and point the plugin at that. This will work.