Non conosco jsp, ma la pagina che fa il controllo dovreebbe essere così
codice:
<%@ page language="java" contentType="text/plain; charset=ISO-8859-1" pageEncoding="ISO-8859-1"%>
<%@page import="java.sql.*,java.util.*"%>
<% String email=request.getParameter("email_field");
            try{ 
                     Class.forName("com.mysql.jdbc.Driver");
                     Connection con = DriverManager.getConnection("jdbc:mysql://localhost:3306/boxit", "root", "");
                     Statement statement = con.createStatement() ;                
                    ResultSet resultset = statement.executeQuery("select email from boxer where email='"+email+"'");                                
                    if(!resultset.isBeforeFirst()) {                    
                            out.print("yes");                
                    }                                


                    else{                    
                            out.print("no");                
                    }            
              }            
              
              catch(Exception e)
              {                
                     System.out.print(e);                
                     e.printStackTrace();            
               }
%>