If you migrated from Exchange 2003 to Exchange 2007 and still have some residual mailboxes left behind on the old exchange 2003 server, this Power Shell cmdlet will be very useful to you.  It will list all users the old exchange 2003 server and their respective mailstores.

Get-Mailbox -OrganizationalUnit OUNAME -Server OLDSERVER | Format-List Alias, ServerLegacyDN

Where OUNAME is an Organization Unit in your Active Directory domain that you want to search.  For example, lets use the users OU.

The OLDSERVER is the name of your old Exchange 2003 server.  For example, let use EX2K3.

So… your command will be like this:

Get-Mailbox -OrganizationalUnit Users -Server EX2K3 | Format-List Alias, ServerLegacyDN