Few things I would check:
- Does this problem happen only in the local server, but not when deployed?
- Are you sure you are NOT including the Javamail JAR files in the classpath? (Appengine SDK has the Javamail stuff embedded, which is what
you should use)
- Are you sure the sender ID (which is "email@domain.com" in your example) has administrative permissions, at least in a 'viewer' role?