Basic Postfix/MS question: avoid scanning mail for invalid recipients

[Dave Jenkins]

Hi,

We have Postfix/MS/SA running on our main & backup mail servers. The
main server is both incoming MX and outgoing, auth required. We'd like
every mail to a valid recipient to be scanned, but don't want to waste
CPU scanning mail to invalid recipients.

Main MX: am I right in thinking Postfix will remove mail to invalid
recipients before it gets to MS & so that setting MS to scan
everything will achieve what we want?

Backup MX: I've written a script to turn /etc/postfix/virtusertable
from the main MX into a valid rules file to be rsync'd to the backup
MX's rules directory, is that a sensible approach? Does MS need to be
restarted when the rules files are updated?

Thanks and sorry for the basic nature of the question,

Dave

CentOS 5.2
postfix-2.3.3-2
mailscanner-4.69.8-1
spamassassin-3.1.9-1.el5