Email Server

Mutt is one common one:

http://www.mutt.org/

A number of my installations use Red Hat 9 and Fedora Core 4 with Sendmail, supporting IMAP and POP. You can also run Fetchmail which collects mail from external mailboxes. Procmail allows you to filter messages before delivery or redirection. You can also run Spamassassin and ClamAV antivirus with Sendmail.

If the company uses Outlook as their email client, have a look at Open-Xchange 5 at http://www.open-xchange.com. It has both a commercial and Open Source versions, and its Outlook Oxtender supports calendars, tasks and shared folders. The product supports Palm, too.

From http://www.open-xchange.com/EN/product/livecd.html you can download a complete server which runs from Linux Live CD.

A server should be a secure one:

courier-mta
postfix
exim
qmail

to name a few. All can be configured using
maildirs. (qmail can only do maildirs)
Maildirs are preferred above mailfiles because
there is no need for locking. They can also be
used with networked devices (nfs, cifs/smb).

Imap server:

courier-imap knows about maildirs.

Then there is the question do you need to
integrate spam/.virus checking?

In that case postfix/exim can handle the
connection with f.e. amavisd-new more elegant
than the other two. (you will also need clam-av
(or other virus checkers) and spamassassin
besides amavisd-new to handle mail checking.

squirrellmail can be used as webmail frontend,
and all imap/pop clients can be used with this
setup.

Using
ldap/mysql/postfix/courier-imap/squirrelmail/amavisd-new
you can build an environment that is completely
configured through a webinterface:

http://www.gentoo.org/doc/en/virt-mail-howto.xml

The system describes the procedure for gentoo,
there emerge is like rpm on fedora (only it
builds from sources).

HIH,
Kind regards,