Oracle Collaboration Suite.
With just about any solution you can use LDAP address books.
using IMAP or POP3 is just something that you're going to have to do if you're not using exchange. and there are POP3 servers and IMAP servers available for free.
and your LDAP server can be something like Linux or Solaris which can both be downloaded and used for free.
Client software can be quite easily obtained for free.
Calendaring is a bit different. I don't think that there are any cheap/free solutions out there that integrate with email. there are however plenty of free/cheap web calendaring solutions. that for a lot of people work well, if not better than exchange calendars, (they can be installed on free platforms, easily accessible externally, not client dependant etc).
basically you can get all the 'features' that are available in exchange for free with a few tools. and you can put them on free platforms too.
the real costs that come with these solutions are the support costs.
I mean take address books, if you install an LDAP server in your organisation, how much experience do you or the technical staff have with this? if you're planning to put this on an open solaris platform then how much Unix expertise have you got available?