1) remove sendmail and replace it with qmail
2) do the following to configure qmail for "maildir" rather than "mailbox" storage:
as the user:
% maildirmake $HOME/Maildir
% echo ./Maildir/ > ~/.qmail
Create a maildir in the new-user template directory and (probably don't need to do this, but I'm keeping it logged here anyway) replace ./Mailbox with ./Maildir/ in /var/qmail/rc.
3) fire up qmail
4) install Courier-IMAP as the imap server.
both packages are available from Freshmeat, and so far seem to work much better than the defaults from redhat. Courier-IMAP mentions the need for "maildrop", but you don't need it as long as you setup Maildir support with Qmail.
This, of course, breaks mail readers like elm, but gives a better imap tool. Pine can read imap, and there's a tool in the qmail package that can convert maildirs so they can be read by elm, etc.