Despite a nodding acquaintance with the rest of the world, MSExchange has always been modeleld on x400. There are other implementations which based around x400 out there (Lotus Notes, and until it was discontinued, Samsung Contact). However IMHO they are all a complete PITA to manage and should be avoided if at all possible.
And also a mail client?
Lets deal with that first. Presumably you mean a client which will provide the users the functionality they have in Outlook. Well, Outlook is probably a sensible starting point. Although Microsoft tend to hide this information away, it will happily talk SMTP, POP, IMAP, LDAP and iCAL.
Indeed it probably makes a lot of sense to plan an LDAP system (or extending your current provision) to provide address book functionality. If you don't already have a suitable LDAP server, have a look at GOSA. Note that if you have multiple sites, then you should really really have LDAP based mail routing set up.
Of course, using open standards for server protocols mean that you can choose from lots and lots of user agents - e.g. you could easily add web based clients for mail and calendar, e.g. squirrelmail, Mozilla calendar (but there are many, many more).
Serverside you need to have a Mail Transport Agent - several years ago I did some research on this and found that Sendmail and Postifx were significantly more effective at sending email compared with Qmail, Exim and others (MS Exchange was bottom of the reliability list). Although Novell's groupwise proved just as reliable.
I'd recommend using an IMAP server in preference to a POP server. There's lots of good ones out tere scalable up to massive levels. The UoW one usually comes bundled in distributions but you probably need something a bit more sophisticated if you have more than 200 or so users - dovecot or courier maybe.
There was an open-source project attempting to reimplement mapi as a standard API but AFAIK its not been very active lately.
In addition to the products mentioned by others, you might want to have a look at sogo
Your not going to get all the answers here - you're still going to have to do some research and probably try integrating different products - but the maintenance effort will be massively reduced. There's some links below to get you started.
http://support.microsoft.com/kb/291621 - iCAL stuff
http://technet.microsoft.com/en-us/library/cc179232%28office.12%29.aspx - LDAP stuff