各位XDJMS~javax.servlet.ServletException: Servlet execution threw an exception

type Exception reportmessage description The server encountered an internal error () that prevented it from fulfilling this request.exception javax.servlet.ServletException: Servlet execution threw an exception
root cause java.lang.NoClassDefFoundError: javax/mail/Session
receive.POP3.execute(POP3.java:20)
org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:431)
org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:236)
org.apache.struts.action.ActionServlet.process(ActionServlet.java:1196)
org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:432)
javax.servlet.http.HttpServlet.service(HttpServlet.java:710)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
note The full stack trace of the root cause is available in the Apache Tomcat/6.0.13 logs.我用JAVAMAIL接收邮件。这是什么错呢？？？？？？？？？？？？？跪求。。要不我的毕业设计就做不完哒。