Of course, you need a client that knows about archive mailboxes before you can use the feature. See this EHLO post for some information on how this setup works. All of this depends on the deployment of Exchange 2010 SP1 as the basic platform for Exchange Online within Office 365 this work is ongoing and Office 365 is currently in beta. In this scenario, the primary mailbox will be located in a database running on an “on-premise” server and the user will connect to a database running on a server in a Microsoft datacenter whenever they want to access the contents of their archive.

Office 365 then offers the prospect of allowing users to maintain their archive mailboxes “in the cloud”. Matters are substantially improved in Exchange 2010 SP1 as it supports the separation of a user’s primary mailbox and their archive mailbox across different databases, giving administrators the ability to consider schemes such as dedicated archive databases or even dedicated archive servers. The introduction of archive mailboxes is one of the major new features offered by Exchange 2010.