JavaScript Email Validation

function validate_Email()
{
// Author: Surendra Sambana
//Purpose: Email validation using java script
// One can improve this using regular expressions.
var strEmail = document.getElementById('EmailId Field');
var len = strEmail.length;
var i_at = strEmail.indexOf("@");
var i_dot = strEmail.indexOf(".");
var i_space = strEmail.indexOf(" ");

if(len < 5) //a@b.c minimum length
{
alert("please enter your E-mail address");
strEmail.focus();
}
else if(i_at<=0 || i_dot<=0 || i_space!=-1 || i_dot==i_at+1 || i_dot==len-1 || i_at==len-1)
{
alert("please enter a valid Email ID");
strEmail.focus();
}
}