Exchange 2007 and 2003/2000 co-existence
Check this article about co-existence 2007 and 2003/2000 systems.
Note the tools usage section:
So which tools to use on which objects? Here's the easy list to remember:
Exchange 2007 mailboxes must be managed with Exchange 2007 management console or shell.
Exchange 2007 mailboxes MUST NOT be managed with Exchange 2003 tools. Note that this is not blocked, but mailboxes managed from Exchange 2003 ADUC will not be fully functional.
Exchange 2003 mailboxes can be edited or removed with Exchange 2007 tools, but cannot be created by Exchange 2007 tools.
Exchange 2003 mailboxes can be managed with Exchange 2003 tools.
Both Exchange 2003 and Exchange 2007 mailboxes can be moved (in either direction) with the Exchange 2007 tools. Exchange 2003 move mailbox cannot be used to move mailboxes to or from Exchange 2007 mailbox server.