The problem with this setup is that spam mail is still delivered, and I need Thunderbird's filters to weed out incoming mail. And when I'm using my webmail (running SquirrelMail), my inbox is a tsunami of unread [SPAM] messages.
So I'd like to go a step further and delete all messages flagged [SPAM] directly on the server. It doesn't look like Spamassassin provides this functionality.
Spamassassin just tags things, it doesn't interact with the final mail delivery. What you need is some form of server-side filtering
My (limited) expertise in MTAs is with Exim and that has an extensive filter syntax that goes in the user's .forward file - I use that to pre-filter my mail before the MUA sees it. From my .forward file:
if $h_X-Spam-Flag: contains "Yes" then save Maildir/.spam/ finish endif
if $header_subject: contains "[CentOS" then save Maildir/.CentOS/ finish endif
If you don't want to use a different MTA, then the classic way of doing server side filtering is using procmail. I'm certain there will be lots of HowTos out there on how to set it all up with Postfix.
Finally, I would be quite surprised if there is no built-in filtering of some form with Postfix, but from the articles I've just been looking at it doesn't seem to be a simple process.
P.