I definitely recommend
daemontools for that. It's part of a full qmail installation. Look for one of the several qmail installation recipes available everywhere (Qmail Rocks is a good one) and follow the instructions, just as much as daemontools is involved. Or rather just follow the instructions on its
home page, it should be easier.
Daemontools is the component that makes sure qmail is always running. I use it for qmail and a couple other unrelated services. You don't need to install qmail for what you want, just daemontools.