Code:
<html><head>
<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>
</head>
<body>

<form name="frmSample" method="post" action="#" onSubmit="return ValidateForm()">
                <p>Enter an Email Address : 
                  <input type="text" name="txtEmail">
                </p>
                <p> 
                  <input type="submit" name="Submit" value="Submit">
                </p>
              </form>
</body>
</html>