I really think this used to be a problem. Why anyone in their right mind would even consider using Sendmail these days, is beyond me. I use exim - because I've used at work, and it's default in Debian -- but I'd generally recommend either postfix, or probably openbsd's opensmtpd (it's in Debian unstable). It's not really that hard, AFAIK no sane distros will set up an open relay out of the box anymore, and that's really all there is to it. Assuming you're serving less than a thousand users or so (eg: one user ;-).

Might want to set up greylisting, though, to curb incoming spam.