Javascript email validation

Hi,
Any body who nows an efficient email validation algorithm in javascript.

HTML Code:

---

<script language = "Javascript">
function echeck(str) {

var at="@"
var dot="."
var lat=str.indexOf(at)
var lstr=str.length
var ldot=str.indexOf(dot)
if (str.indexOf(at)==-1){
  alert("Invalid E-mail ID")
  return false
}

if (str.indexOf(at)==-1 || str.indexOf(at)==0 || str.indexOf(at)==lstr){
  alert("Invalid E-mail ID")
  return false
}

if (str.indexOf(dot)==-1 || str.indexOf(dot)==0 || str.indexOf(dot)==lstr){
    alert("Invalid E-mail ID")
    return false
}

if (str.indexOf(at,(lat+1))!=-1){
    alert("Invalid E-mail ID")
    return false
}

if (str.substring(lat-1,lat)==dot || str.substring(lat+1,lat+2)==dot){
    alert("Invalid E-mail ID")
    return false
}

if (str.indexOf(dot,(lat+2))==-1){
    alert("Invalid E-mail ID")
    return false
}
               
if (str.indexOf(" ")!=-1){
    alert("Invalid E-mail ID")
    return false
}

return true                                       
}

function ValidateForm(){
var emailID=document.frmSample.txtEmail
       
if ((emailID.value==null)||(emailID.value=="")){
  alert("Please Enter your Email ID")
  emailID.focus()
  return false
}
if (echeck(emailID.value)==false){
  emailID.value=""
  emailID.focus()
  return false
}
return true
}
</script>

---

There is a regular expression designed for this ... I would have to search for it, but with olivthill's code, there exist some email addresses that are valid that will be told are invalid.