can you post the php code you're using to check and send the email?
Also: you create a variable named lvOut, which is a LoadVars object, but then you use a variable named lvOutOut, which isn't even defined. try using this code:
ActionScript Code:
var lvOut:LoadVars = new LoadVars ();
lvOut.email = txt.text;
lvOut.sendAndLoad ("http://www.mysite.nl/newsletter/form.php?form=1", "POST");
lvOut.onLoad = function (success:Boolean) {
// process the returned data
}