John,
Unfortunately your fix doesn't work. All high-scoring spam with empty return-path<> is stored in my regular mailbox again 

"regular" spam still can be found in the spambox.
A few headers (might be usefull?):
Return-path: <>
X-Spam-Checker-Version: SpamAssassin 3.3.2 (2011-06-06) on <myserver>
X-Spam-Flag: YES
X-Spam-Level: ************
X-Spam-Status: Yes, score=12.8 required=4.6 tests=BAYES_99,FSL_HELO_BARE_IP_1,
	FSL_HELO_BARE_IP_2,MISSING_DATE,MISSING_MID,RCVD_IN_BRBL_LASTEXT,
	RCVD_NUMERIC_HELO,RDNS_NONE autolearn=no version=3.3.2
X-Spam-Report: 
	*  1.4 RCVD_IN_BRBL_LASTEXT RBL: RCVD_IN_BRBL_LASTEXT
	*      [37.212.79.65 listed in bb.barracudacentral.org]
	*  3.5 BAYES_99 BODY: Bayes spam probability is 99 to 100%
	*      [score: 1.0000]
	*  1.7 FSL_HELO_BARE_IP_2 FSL_HELO_BARE_IP_2
	*  2.3 FSL_HELO_BARE_IP_1 FSL_HELO_BARE_IP_1
	*  1.2 RCVD_NUMERIC_HELO Received: contains an IP address used for HELO
	*  0.5 MISSING_MID Missing Message-Id: header
	*  0.8 RDNS_NONE Delivered to internal network by a host with no rDNS
	*  1.4 MISSING_DATE Missing Date: header
Any new thoughts on this?
Danny