I think I found a little bug in the Newsletter.
Since it uses the default Silverstripe mail-classes , it is also taking into account the _config.php mail-settings.
For me , this meant I just received a couple of thousand bcc mails because the client just sent a newsletter... Haha..
It might be wise to disregard the _config.php bcc_all_emails_to and cc_all_emails_to for the newsletter module...
Let me know what you think.