Managed Microsoft Exchange Service

We currently manage mailboxes on behalf of over 20,000 users. We are about to launch a managed Microsoft Exchange 2003 service in the UK - the servers will be hosted either on the customer own site [and managed remotely] or in our data centre.

We are keen to capture ideas on what the most critical service requirements would be for this type of solution [eg: Monitoring, System Management, Gateway services to OWA etc.]

Any ideas ? Comments ?

Thanks