function isEmail(listname,formNR,messageText)
// kijkt of een string een e-mail adres is 
// inputs:  emailAddress [string]
// outputs: true of false [boolean]
{
    
        for (var i = 0; i < listname.length; i++)
        {if (!stringEmpty(document.forms[formNR].elements[parseInt(listname[i])].value))
        
            {//alert ((document.forms[0].elements[parseInt(listname[i])].value.indexOf ('@') == -1))
                   if (document.forms[formNR].elements[parseInt(listname[i])].value.indexOf ('@') == -1 || document.forms[formNR].elements[parseInt(listname[i])].value.indexOf ('.') == -1 || document.forms[0].elements[parseInt(listname[i])].value.indexOf(" ",2) >= 0) 
                      {
                        alert (messageText)
                        document.forms[formNR].elements[parseInt(listname[i])].focus()
                        return false;
                        break
                   
                      }
                        
            }
        }



return true
}