the record set being used in the email is filtered on the "NieuwsbriefID" query string variable.
However, the forms action is blank, so does not pass that variable when the form posts:
<form action="" method="post" name="emailings" id="emailings">
change that to:
<form action="emailing.asp?NieuwsbriefID=<%= Request.QueryString("NieuwsbriefID") %>" method="post" name="emailings" id="emailings">