function checkmailaddress(str) {
// Set variables
    var at="@"
    var dot="."
    var lat=str.indexOf(at)
    var lstr=str.length
    var ldot=str.indexOf(dot)

// Check that first string entered is valid
	if (str.indexOf(at)==-1){
       alert("This is NOT a invalid E-mail address!")
       //String is not valid
	   return false
    }
// Check that strings entered are valid
    if (str.indexOf(at)==-1 || str.indexOf(at)==0 || str.indexOf(at)==lstr){
       alert("This is NOT a invalid E-mail address. A valid email address looks like this ' billme@yahoo.com '. Please try again!")
		//String is not valid
        return false
    }
// Check that strings entered are valid
    if (str.indexOf(dot)==-1 || str.indexOf(dot)==0 || str.indexOf(dot)==lstr){
        alert("This is NOT a invalid E-mail address. A valid email address looks like this ' billme@yahoo.com '. Please try again!")
		//String is not valid
        return false
    }
// Check that strings entered are valid
     if (str.indexOf(at,(lat+1))!=-1){
        alert("This is NOT a invalid E-mail address. A valid email address looks like this ' billme@yahoo.com '. Please try again!")
		//String is not valid
        return false
     }
// Check that strings entered are valid
     if (str.substring(lat-1,lat)==dot || str.substring(lat+1,lat+2)==dot){
        alert("This is NOT a invalid E-mail address. A valid email address looks like this ' billme@yahoo.com '. Please try again!")
		//String is not valid
        return false
     }
// Check that strings entered are valid
     if (str.indexOf(dot,(lat+2))==-1){
        alert("This is NOT a invalid E-mail address. A valid email address looks like this ' billme@yahoo.com '. Please try again!")
        //String is not valid
		return false
     }
 // Check that strings entered are valid   
     if (str.indexOf(" ")!=-1){
        alert("This is NOT a invalid E-mail address. A valid email address looks like this ' billme@yahoo.com '. Please try again!")
		//String is not valid
        return false
     }
	//String is valid
     return true          
  }

<!-- =================JavaScript Function ValidateForm validates input of EmailRemove==================== -->  
function ValidateForm(){
  var mailaddress=document.EmailRemove.txtEmail
  
  /* Check field value for valid input*/
  if ((mailaddress.value==null)||(mailaddress.value==""))
  	{
    	alert("This is NOT a invalid E-mail address. A valid email address looks like this ' Jeffrey@yahoo.com '. Please try again!")/* Alert if field has no valid input*/
    mailaddress.focus()/*set focus on mail address field*/
    return false
  }
  /*Check for valid mail adress*/
  if (checkmailaddress(mailaddress.value)==false){
    mailaddress.value=""
    mailaddress.focus()  /*Check for valid mail adress*/
    return false
  }
  return true
 }