zevsek,
You'll find a few web-based email systems at
cgi.resourceindex.com/Programs_and_Scripts/Perl/Web_Based_E_Mail/ , but there's several things you should know. One, most of the web-based mail systems are very expensive. Two, most of them are really only interfaced to POP3 accounts already set up, so you'll still have to do the work of setting up the accounts.
The only solutions I've seen that will set up the POP3 accounts for you are run via cron (which is a bit OTT) or VERY expensive.
Possibly another way to do it is to use one of the services that offer to host your web-based mail for you, but again, this can be prohibitive, because your host will have to agree to having a canonical routed to another provider.
Anyway, the best products are Endymion
www.endymion.com (pretty, but slow), CWMail (Don't know the URL, fast, but ugly) and EmuMail (bit of both, but expensive).
However, if you find somthing better out there, please drop a note onto this board, as I've tried them all and I still haven't found a happy medium...
adam