Google Apps for Your Domain
Google provides great email service via gmail. However, they also provide a whole bevy of services for small and large businesses, alike. Check out Google Apps:
Google Apps provides email, shared calendaring, shared contacts, and an online document workspace where you can create documents, spreadsheets, and presentations - all compatible with Microsoft Office. The best part is that all of this is available with a web browser - no client software is necessary.
For larger enterprises, Google Apps provides email filtering, spam blocking, and archive retention for compliance purposes at very reasonable rates.