JuanCi
I recommend search smtp protocol info
You must end each line with \r\n
You must end you message with a dot \r\n.\r\n
import java.net.*;import java.io.*;
public class nombreIP{
//Streams de comunicación
DataInputStream is; DataOutputStream os;
public nombreIP(){
System.out.println("declaro sk"

;
try{//Abrir socket
Socket sk=new Socket("smtp.server.com", 25);
os=new DataOutputStream(sk.getOutputStream());
is=new DataInputStream(sk.getInputStream());
//Respuesta del servidor
System.out.println(is.readLine());
//Me identifico
String local;
local=InetAddress.getLocalHost().toString();
os.writeBytes("HELO "+local+"\r\n"

;
System.out.println(is.readLine());
//Datos mensaje
os.writeBytes("MAIL FROM:<remite@dominio.com>\r\n"

;
System.out.println(is.readLine());
os.writeBytes("RCPT TO:<destino@dominio.com>\r\n"

;
System.out.println(is.readLine());
os.writeBytes("DATA\r\n"

;
System.out.println(is.readLine());
//Poner aquí el mensaje
//Cada salto de linea, \r\n
os.writeBytes("Your message\r\n"

;
//Final mensaje
os.writeBytes("\r\n.\r\n"

;//Valor fijo
System.out.println("Fin del Mail."+is.readLine());
}
catch (Exception ex){System.out.println("Mensaje no enviado\n"+ex);}
}
public static void main(String ar[])
{nombreIP INICIO=new nombreIP();}
}