What are the needs that this server can address?

At this time the church’s website is hosted by powweb on their free package for non profits.  Unfortunately their e-mail services are horrendously unreliable.  We have a business class internet connection with a static ip.  We have more than enough outgoing bandwidth to handle e-mail.  While Google apps are available I don’t trust the cloud enough to host a NPO’s e-mail.  Now the decision is what mail server am I going to run?  Ms Exchange is one option…however i don’t want to be handcuffed to the office/outlook/exchange lock-in.  I am a huge fan of Linux and I use it everywhere I can.  I need the ability for the users to have Exchange functionality without having to be handcuffed to Outlook.  Ideally I would want to do everything inside a web browser.  There’s several options just read the list here.  I don’t want to pay for a software package that runs on top of a free operating system.  I also require the software to integrate with my currently installed Active Directory.  Out of all of the choices I chose Zimbra.  I made this choice due to the cost, available features, low system requirements, and overall reliability.  The ability to use shared contacts, calendar, and even inboxes are available in the free version.   I know of one large deployment at my wife’s employer…while it wasn’t done in a particularly good way it still just works.  While Zimbra has it’s own anti-spam and a/v I’m going to use my firewall’s spam and malware filtering as it’s quite powerful and already tweaked.  Also everything at a base level is done through a web browser which means i don’t have to use the outlook monstrosity.  While the interface is different retraining isn’t going to be that painful.