I guess this is meant to "todgerme" the howto author of "How To Fight Spam Using Your Postfix Configuration".
I have a working Mail Server which I configured using the "Virtual Users And Domains With Postfix, Courier And MySQL (+ SMTP-AUTH, Quota, SpamAssassin, ClamAV)", and I wanted to add some additional spam filtering and I happen to read your howto tutorial and I tried configuring the said Mail Server.
After the configuration, I encountered this problem.
Dec 21 11:54:20 mail postfix/master: warning: process /usr/lib/postfix/virtual pid 5300 exit status 1
Dec 21 11:54:20 mail postfix/master: warning: /usr/lib/postfix/virtual: bad command startup -- throttling
Dec 21 11:55:20 mail postfix/virtual: fatal: mysql:/etc/postfix/mysql-virtual_mailboxes.cf: proxy map is not allowed for security sensitive data
I hope you can enlighten me and the rest on this matter.