Actually, you didn't mention how big the hard drive on the old Pentium is, but I assume it's small since you added your question to this thread. Just how small is it?

And is there some reason you really need Knoppix for your mail server project? Most of the stuff that makes Knoppix so big is aimed at either desktop/workstation use or diagnostic/utility use. It's a little lacking in server applications, but at the same time loaded down with stuff inappropriate to server use. Same for the hardware autodetection--most users don't want it in a server after the initial install. So if your interest in Knoppix is simply to get a usable Linux on a system, you might look into the Knoppix-based Morphix we mentioned earlier. For server use, if you install Knoppix you may want to change your default runlevel from 5 to 2, so you don't run the GUI. With Morphix, I think that can be specified at install.

As far as e-mail, Knoppix installs something called smail. I am not familiar with it, but it might work for what you want. What I KNOW will work is sendmail. I suppose it has a reputation for being difficult to use, but there is a lot of documentation available, and Webmin has a very good interface for managing it. And it is easy to install with apt-get, after removing smail. (QMail is supposedly easier, and also is supported by Webmin. But there might not be a .deb for it.)

If it's just running a mailserver, you might look at SME Server. It's designed to be a sort of "server appliance", doing routing for Internet access, mail, DNS, DHCP, and a Samba server--but you can turn most of it off. The nice thing is it has a webmail application built in.