It is a tool for reengineering 3rd party, closed, binary Android apps. It can decode resources to nearly original form and rebuild them after making some modifications; it makes possible to debug smali code step by step.
K9 is an opensource email client based on the Email application shipped with the initial release of Android. K9 is focused on making it easy to chew through large volumes of email. It's our hope that K9 leads to improvements in the core Android mail client.