Ray,
I see the same error message occasionally when retrieving email, however, because I am able to retrieve email when retrying in a few minutes, I have not gone through a troubleshooting exercise. To troubleshoot, you would need to capture requests on the Exchange server side and then see why the Exchange server refuses requests occasionally. This troubleshooting would take place outside of Mendix.
As for your app store modules, look in your Project node in the Project Explorer. App Store modules are all stored there in Mendix 7.
Mike