Algo así te sirve?
Código:
<html>
<head>
<title> New Document </title>
<script language="JavaScript">
<!--
function checkForm(frm){
if (frm.nombre.value == ""){
alert("Ingrese nombre");
frm.nombre.focus();
return false;
}
if (frm.mail.value == ""){
alert("Ingrese mail");
frm.mail.focus();
return false;
}
frm.recipient.value = frm.mail.value + "@dominio.ext";
}
//-->
</script>
</head>
<body bgcolor="#FFFFFF">
<form method=post action="http://www.melodysoft.com/cgi-bin/form.cgi" onSubmit="return checkForm(this);">
<input type="hidden" name="recipient">
<input type="hidden" name="subject" value="Mail de Prueba">
Nombre: <input type="text" name="nombre"><br>
Mail: <input type="text" name="mail"><br>
<input type="submit" value="Enviar">
</form>
</body>
</html>
<hr noshade size=1><img src="http://www.gograph.com/Images-8712/ClipArt/cat03.gif" height="50" border=0 align="absmiddle"> <font size="2" face="verdana" color="#000000">
bet[/CODE]