Based on the documentation NoClassDefFoundError is
Thrown if the Java Virtual Machine or a ClassLoader instance tries to load in the definition of a class (as part of a normal method call or as part of creating a new instance using the new expression) and no definition of the class could be found.
The searched-for class definition existed when the currently executing class was compiled, but the definition can no longer be found.
Couple of things you should do to make sure all the classes are available at runtime.
1 Make sure that you add Java Mail API to your classpath.
2 Make sure that you are using the -classpath option to pass the classpath to java as well as to javac to pick up those .jar files at runtime:
for example:
java -classpath :.:/usr/jdk/mail.jar