When using a Remote Users Collection and migrating mailboxes to the target Exchange 2010 the MSA needs to delete and then to create the mailbox, and the MSA might report the following.
Any agent dealing with Exch 2010 requires the Exchange 2010 Management Tools to be installed. This is outlined in the Release Notes, under System Requirements. In this rare situation, when MSA is creating target mailboxes, the agent host where the MSA is installed, requires Exchange 2010 Management Tools to be installed.
In order for the MSA to process a remote collection where the target mailbox is on Exchange 2010, the Exchange 2010 Management tools must be installed on the server running the MSA.
However, if the MSA is installed on source Exch server, and source consists of Exchange 2007 Servers running Windows Server 2003, then the Exchange 2010 Management Tools cannot be installed.
The Exchange 2010 Management Tools can only be installed on Windows Server 2008, Windows Server 2008 R2, Windows Vista, and Windows 7, and the machine must be 64 bit.
Trying to find a Windows 2008 Server in the source forest and trying to get the Exchange Management Tools installed could be problematic, because the installation process wants to verify that the forest schema is up-to-date and wants the installer to be an Enterprise Admin and wants to connect to the Active Directory of the source forest.
One of the most elegant workarounds in such situation is to install an additional mailbox source agents on one of the Exchange 2010 mailbox servers in the target forest. In addition, since Exchange 2010 uses Client Access Servers for MAPI connectivity, it may perfect sense to install the MSA on servers that have the client access server (CAS) role. Another reason for installing MSA directly on CAS server is the fact, that during OST mailbox migration there are no PRV files being created and MSA will have to use a big amount of data during a short period of time.
Ensuring that MAPI CDO is already installed, the MSA's will install just fine on the target CAS server.
Also, important to note is that the MSA needs to connect to the source Exchange server using the NetBIOS name of the server. This isn't a problem when the MSA is in the source forest, because the servers are typically configured in DNS to append the local domain. However in a cross-forest scenario, an admin will have to ensure that the source server name can be resolved from the target server. This can be done by putting the source forest's domain name in the DNS suffix search order, or by ensuring that the host names and FQDN's of the source mailbox servers are in the host files on the agent host which is hosting the MSA (in this case the Exchange 2010 Client Access Server).
The information provided here is not a guideline and not an official recommendation, rather an attempt to make the decision easier and to mention the main aspects.