I had to do the following to make newsletter module send mails through smtpmailer correctly
Silverstripe 2.3.3
SmtpMailer [v0.1.0]
Newsletter [v0.4.0-rc1]
in smtpmailer/code/SmtpMailer.php line 43 change
$this->sendMailViaSmtp( $to, $from, $subject, $attachedFiles, $customheaders, false );
to
return $this->sendMailViaSmtp( $to, $from, $subject, $attachedFiles, $customheaders, false );
Hopefully this saves time for others ;-)