eMail Validation Scripts

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Javascript eMail validation script

Here is the code:

<script language = "Javascript">
function emailcheck(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("The eMail address '@' convention appears to be invalid.")
return false
}
if (str.indexOf(at)==-1 || str.indexOf(at)==0 || str.indexOf(at)==lstr){
alert("The eMail address '@' convention appears to be invalid.")
return false
}
if (str.indexOf(dot)==-1 || str.indexOf(dot)==0 || str.indexOf(dot)==lstr){
alert("The eMail address 'dot' convention appears to be invalid.")
return false
}
if (str.indexOf(at,(lat+1))!=-1){
alert("The eMail address '@' convention appears to be invalid.")
return false
}
if (str.substring(lat-1,lat)==dot || str.substring(lat+1,lat+2)==dot){
alert("The eMail address 'dot' convention appears to be invalid.")
return false
}
if (str.indexOf(dot,(lat+2))==-1){
alert("The eMail address 'dot' convention appears to be invalid.")
return false
}
if (str.indexOf(" ")!=-1){
alert("The eMail address spacing convention appears to be invalid.")
return false
}
return true 
  }
  function ValidateForm(){
  var emailID=document.myForm.txtEmail
  if ((emailID.value==null)||(emailID.value=="")){
  alert("Please Enter your Email Address")
  emailID.focus()
  return false
  }
  if (emailcheck(emailID.value)==false){
  emailID.value=""
  emailID.focus()
  return false
  }
  return true
  }
</script>
<form name="myForm" method="post" action="" onSubmit="return ValidateForm()">
  Enter an eMail Address : 
<input type="text" name="txtEmail"> 
<input type="submit" name="Submit" value="Submit"> 
</form>