Problem

Description	Resource	Path	Location	Type
The  App Engine SDK 'App Engine' on the project's build path is not valid.	Mail		Unknown	Google App Engine Problem

SDK directory '/Applications/eclipse/plugins/...334/appengine-java-sdk-1.2.0' does not exist

Solution

Reinstall GWT and GAE in Eclipse (by uninstalling them, and then installing; and don't forget to restart eclipse each time)

Then go to the project properts and configure google SDK's once again

To create eclipse project make sure to run from the samples applications before you try to import it into eclipse

../../webAppCreator -ignore com.google.gwt.sample.mail.Mail