After successfully building the beta4 Mail sample, I get the following error when I run it:
[ERROR] Unable to instantiate 'com.extjs.gxt.samples.mail.server.MailServiceImpl'
java.lang.ClassNotFoundException: com.extjs.gxt.samples.mail.server.MailServiceImpl
This is on OS X 10.5.3 with java version "1.5.0_13".