My suggestion for that heavy load would be qmail+vpopmail. Here's a link to a toaster that should help you out:
Everything you need is there, including the web-based access. It is a bit dated though, but unless you're using glibc 2.3.1 or greater, you should be fine. If you are using glibc 2.3.1 or greater, then you will need to patch a couple of the qmail packages with the errno patches, which can be found on http://www.qmail.org/top.html.
I'd also recommend reading Life with Qmail
prior to ever starting. Qmail can be a pain to set up, but once you have it set it properly you can forget about it. It's rock solid, very secure, and more than capable of handling your load.