I admit I'm pretty new to things, so I don't exactly know what I need or what to even look for.

I'd like to offer a webmail interface to people who I can give pop3 accounts to. I'm running a VPS with cpanel if there's something that's maybe built-in that I don't know about.

I don't need anything fancy, but several of the people I run the site with don't use email clients and I don't know any other way besides a webmail interface to get them to be able to use their email addresses @mysite.com. Plus I wouldn't mind being able to check my own mail while I'm out of town and stuff.

I'd also like for one installation/instance to be able to cover multiple domains if possible.

what should I be looking for and installing to make this happen, and is it gonna be terribly difficult?

DWMail from Dominion Web. www.dominion-web.com. You can lock it down to certain domains or allow any pop 3 account to be checked. I dont know if they havr installation support. The install was moderately difficult.

There are open-source solutions like horde or squirell mail which are more basic applications. It was pretty easy to brand dwmail.

I like horde alot. you can try squirl mail as well its not too bad.

www.squirrelmail.org is indeed a good basic application.