I'm using CentOS with a "Parallels Plesk" installation, a default qmail configuration and want to restrict email traffic to my virtual domain (no incoming/outgoing mail from/to any external domain). How do I configure qmail/plesk to achieve this?
If you want to prevent your server sending outbound mail to anything but local domains, then disable relaying for all.
There will be a 'cdb' control file that your tcpserver reads when it starts qmail-smtpd and it will be setting the environment variable RELAYCLIENT for any network that is permitted to relay to non-local domains. (See tcpserver(1), particularly the -x option, and also tcprules(1) and qmail-smtpd(8) for the RELAYCLIENT environment part.)
(on my vanilla-ish qmail installation that file lives in /service/qmail-smtpd/cdb, and is built from a file /service/qmail-smtpd/tcp using the tcprules command. Remove every occurrence of RELAYCLIENT="", rebuild the cdb file, and no host will be able relay - they will only be able to send email to domains in "/var/qmail/control/rcpthosts". Double-check that only your domains are in that file, too!)
If you want to prevent other servers sending email inbound to you at all (I'm not entirely clear from your original question, or the clarification, whether you're after this bit as well), then in that same control file after all the local networks that previously had RELAYCLIENT set, put a whole new line ":deny" and everyone else will be denied access completely.