I'm not a great fan of quarantining, although it works fairly well for webmail systems where the quarantine can be accessed through the same interface as the inbox (eg, Gmail and Hotmail). It's less helpful where mail is delivered to a POP3 or IMAP box as users have to go to a separate interface to check the quarantine.
With quarantine and IMAP, one approach is using sieve with IMAP server and forwarding the border cases automatically via sieve rules to users junk/spam folder. That way quarantine can be accessed from the regular mail client or web interface and checked by the user him/herself if important mail seems to be missing.
At least Cyrus can do this pretty painlessly, and I think Dovecot does support sieve these days too.
-Reko
