I'm trying to decide between webmin, ensim, and plesk. I really like WHM/cPanel but there is no way I can afford it. I'm about to get my first dedicated server which will be primarily for my own site, but I also plan to host 10-20 sites for customers I have designed web sites for. I don't really care for plesk, especially the client's interface, and I don't like the way ensim makes it an "ensim" system rather than a RedHat system. Webmin seems like it will do everything I need, leave everything "standard" RedHat, and the price is right for me as I'm getting started.

However, the one thing that would be missing is the ability to allow clients to setup their own email pop boxes, aliases, autoresponders, etc. Is there a good secure 'client control panel' or 'client email control panel' to use in combination with webmin?

I think,
if you're using qmail, you can use qmailadmin. I have not looked at it very extensively but I believe it does what you're looking for.

Most dedicated providers can add PLESK for a small monthly fee, ask them about it. Or, as said above, install Qmail : http://www.qmail.org and use QmailAdmin : http://www.inter7.com/qmailadmin

I'm not sure if I like plesk - it seems good from the server admin point of view, but it doesn't seem to have the fit and finish or any powerful features from a client perspective and seems pretty un-intuitive. Maybe I'm not accessing the demo correctly, but the client plesk panel doesn't seem to offer much at all.

Well, it takes some time to get used to Plesk. After that, it proves to be quite good and powerfull also. Has some downsides too, but don't they all.

Probably the most versatile control panel that I know of is H-Sphere (http://www.psoft.net) and I believe we'll go in that direction soon...

Ales