From what I've seen, qmail setups on Plesk do not lock to a specific IP and listens on all IP's bound to a NIC, but when the smtp server sends email, it will always use the primary IP. If you want to lock email to a specific IP, lock it to the Plesk administrative (primary IP), set all domains's MX records hosted by the box to the primary IP, that should be good enough. If you want to go even further you can play with the firewall to only allow mail to the one IP. In normal Qmail setups there are ways to lock it to a specific IP, not sure with Plesk w/o looking at the control files/rc files myself.