SPF not work as expected

[kram@]

I have had a SPF record on my domain for quite some time.
Recently I started getting 3 - 6 SPAM emails a day all using my email address.
Can anybody tell me what i might be missing here?

AS far as I understand it, the SPF records sould check the sending IP, if not mine, it should deny...

My SPF record is

v=spf1 ip4:xxx.xx.xx.xx mx a:server.domain.co.za mx:domain.co.za ~all

Here is the header of the offending email.

X-Spam-Level:
X-Spam-Status: No, score=-0.4 required=5.0 tests=BAYES_00,HTML_MESSAGE,
MIME_HTML_ONLY,MISSING_MID,RDNS_DYNAMIC,SPF_NEUTRAL autolearn=no version=3.2.5
X-Spam-Report:
* 0.0 MISSING_MID Missing Message-Id: header
* 0.7 SPF_NEUTRAL SPF: sender does not match SPF record (neutral)
* 0.0 HTML_MESSAGE BODY: HTML included in message
* 1.5 MIME_HTML_ONLY BODY: Message only has text/html MIME parts
* -2.6 BAYES_00 BODY: Bayesian spam probability is 0 to 1%
* [score: 0.0000]
* 0.1 RDNS_DYNAMIC Delivered to trusted network by host with
* dynamic-looking rDNS
Received: (qmail 32720 invoked from network); 18 Apr 2010 17:27:07 +0200
Received-SPF: neutral (server.domain.co.za: 79.101.173.106 is neither permitted nor denied by domain of domain.co.za) client-ip=79.101.173.106; [email protected]; helo=[79.101.216.6];
Received: from 79-101-173-106.dynamic.isp.telekom.rs (HELO ?79.101.216.6?) (79.101.173.106)