Has anyone setup a qmail toaster on RH ES 3.0? Should I compile for RH7.3/8/9 or something else? ES 3.0 isn't listed, but isn't it based off 7.3?

It's based on RH 9.

I have no experience with qmail toaster and RHEL combo, but I wouldn't expect any problems.

I use Exim with Red Hat Linux Enterprise, there was a big problem in the begining that you can read more about at http://www.webhostingtalk.com/showthread.php?threadid=221803. I belive the problem was because of the dedicated server company, not because of the incompatibility. Now it's all up and running and there are no problems with it.

Use FreeBSD, that's what djb does. :)