ELSE IF Statement not validating information

Validate form has been working fine but I have now tried adding email validation to the code and now nothing will validate, form submits without any pop up error box's?

Here's The Current Code:

enter code here
<script type="text/javascript">
    function validateForm(){


var a=document.forms["order_form"]["fname"].value;
var b=document.forms["order_form"]["address"].value;
var c=document.forms["order_form"]["city"].value;
var d=document.forms["order_form"]["pcode"].value;
var e=document.forms["order_form"]["email"].value;
var atpos=email.indexOf("@").value;
var dotpos=email.lastIndexOf(".").value;



if (a==null || a=="")
    {
    alert("Full name must be filled out");
    return false;
    }
else if (b==null || b=="")
    {
    alert("Address must be filled out");
    return false;
    } 
else if (c==null || c=="")
    {
    alert("City must be filled out");
    return false;
    }
else if (d==null || d=="")
    {
    alert("Post-Code must be filled out");
    return false;
    }
else if (e==null || e=="")
    {
    alert("Email Address must be filled out");
    return false;
    }
else if (atpos<1||dotpos<atpos+2||dotpos+2>=email.length)
    {
    alert("Not a valid e-mail address");
    return false;
    }
  }
</script>