You can get email server specific distros but there isn't really an 'email server' as such in the Linux/unix world.
What you will be looking to set up is an underlying delivery/reception system for the emails themselves and a method to present them to the clients.
Initially, install an MTA (mail trasport agent) which will look after sending and receiving emails. The omst common is "sendmail" but this can be complex to configure. Some other popular MTAs to consider are "postfix", "exim" and "qmail".
Now you have the MTA set up, you'll need an MDA (mail delivery agent). This receives the mails from the MTA when the MTA realises the mail is for local delivery and not to be forwarded on. The most common implementation for this by far is "procmail" and will be installed as default on virtually all systems. By default, it puts the emails into individual users' spool files.
Finally is the MUA (mail user agent). You can run mail client programs on the linux box (mutt, pine, evolution etc) but from what you say, you'll best use an IMAP4 server (IMAP4 is the 'successor to POP3 but it retains the mails on the server rather than delivering them out perminently to the remote client). A common IMAP server for Linux is the "University of Washington" IMAP server. Most mail clients (outolok, outlook express, eudora etc will read IMAP).
A final alternative is to put a webmail interface on top so your staff can read it via a browser - "squirrelmail" is a good one.