Exchange, Zimbra, or others - assistance is needed

We are looking to replace our Exchange 2003 server.  I'm open to any option at this point.  Here are some things that are important to us:

- delegates - executive assistants sending on behalf, viewing cal, contacts, etc
- shared contacts - these are contacts outside the GAL, common external contacts for use across organization, accessible from smart phones!!!
- multiple domains - we have several separate organizations on the same server
- Outlook integration
- shared calendars

I could go with Exchange 2010, although I would probably go a hosted route.  I could go Zimbra, but I don't know enough about it.

The problems - execs are really used to delegates accessing their accounts.  Users are used to public folders (central calendar and contacts).

Any suggestions?  I just could go with Exchange and be done...but it is just such a beast, which is why I'm looking into alternatives or hosted Exchange.

Anyone have any thoughts?

Thanks

Brian