Setting up Linux as a Mail Server
One of the ways to set up Linux as an email server (there are lots and lots of ways) is as follows:
RedHat 6.2+
Install sendmail
install the imap RPM which contains pop3 and imap
set up inetd.conf or xinetd to allow access to pop3 and imap
# Pop and imap mail services et al
imap stream tcp nowait root /usr/sbin/tcpd imapd
#pop-2 stream tcp nowait root /usr/sbin/tcpd ipop2d
pop-3 stream tcp nowait root /usr/sbin/tcpd ipop3d
Restart inetd (/etc/rc.d/init.d/inetd restart) or (/etc/rc.d/init.d/xinetd restart).
You should now be able to configure a mail client (netscape, outlook, ...) to attach to the mail server either through pop3 or imap.
Setting up Linux as a Mail Client for MS Exchange
Get the latest Fetchmail 5.8.0+
http://www.fetchmail.org
Configure it with the address and login information to your imap exchange server.
Get an LDAP client in order to be able to look at the global address book (netscape, staroffice, kldap, gq, others).
I know this section is very short, but getting it to work is pretty simple - especially with fetchmail.