Spamhaus http://www.spamhaus.org/ has the limit to the DNS queries for email server. This limition hit our email server. Yep we received so much spam the we hit the spamhaus limitation.
We decided to try two methods to increase the efectiveness of our SPAM filtering.
We are using postfix, by putting software like SQLgrey it will help to decrease the numbers of spam that enter into Mailscanner.
The site above provide clamav signatures that will scan for spam in email including inside PDF files.
and now to choose between MailScanner that used Spamassassin and DSPAM.
or we just build our own DNSBL :)