Here is the fix I found for the spam-assasin problem. It fixed the problems I had with contact forms:
mail($email_to, $subject, $message, "From: ".$your_name." <".$youremail.">"."\r\n"."Message-ID: <".md5($youremail)."@yourdomain.com>"."\n".'Content-Type: text/html; charset=iso-8859-1\n Content-Transfer-Encoding: 8bit\n')or die('mail not sent');
The Message-ID evidently is too short coming from your server, or something like that. Using md5 from your email address seems to do the trick quite nicely.
My spamassasin score went down from 5.5 to 1.3 with this little fix.