riddel recommended kolab to jorge and i have to agree.
why? because i'm using it and depending on it and it hasn't let me down yet. i have a kdemail.net account that uses kolab2, as do many other kde developers. the company that i've been consulting to for the last several months is also running kolab2 and they rely on it's calendaring and mail services to keep their business running. what's more, they are an IT services company and are starting to roll out kolab2 to several of their customers based on the success they've had with it. cheaper than exchange and it integrates with outlook, web browsers and kontact for groupware =) spam filter, virus scanning, web admin ... heck, it's even got server replication built in!
as for installation ... if you're running debian it's a 10 minute job to set up using the provided debian packages. throwing kolab on top of kubuntu, for instance, is a dream. if you aren't running debian, it's pretty much the same set of steps but it takes more wall clock time as the install script has to compile the packages from source.