function validEmail(theEntry) {
  badEntry = false
  invalidChars = " /:,;"     
  if (theEntry == "") { 
    badEntry = true
  }
  for (i=0; i < 5; i++)  {
    badChar = invalidChars.charAt(i)
    if (theEntry.indexOf(badChar,0) > -1) {
      badEntry = true
    }
  }  
  atsignLoc = theEntry.indexOf("@",1)
  if (atsignLoc == -1) {
    badEntry = true
  }     
  if (theEntry.indexOf("@",atsignLoc+1) > -1) {
    badEntry = true
  }
  dotLoc = theEntry.indexOf(".",atsignLoc)
  if (dotLoc == -1) {
    badEntry = true
  }
  if (dotLoc+3 > theEntry.length) {
    badEntry = true
  }
  return badEntry
}

function validateForm() {
  var content = "Vul a.u.b. de volgende velden in:\n";
  validity = true;
  var frm = document.mail_form;
  if (validEmail(frm.mailto.value) == true) {
    content = content + "- E-mail adres van ontvanger\n";
    validity = false;
  }
  if (! validity) alert(content);
  return validity;
}