This article looks at who can use ECP while co-existing Exchange 2010 with 2003. A user with no mailbox, a 2003 mailbox user and 2010 mailbox user – all with necessary rights.
Let me state that if you have an Exchange 2010 admin account (member of the RBAC groups) with mailbox on a 2010 server, you can login to ECP and manage your organization.
But, what if you have an Exchange 2010 admin with the mailbox on 2003? The answer is that even if you have the right permissions, the fact that your mailbox is on 2003 blocks the admin from using ECP. This is the case, even if the redirection from OWA 2010 to 2003 works. ECP is purely for 2010 and hence a 2003 user won’t be able to use it. If you do try, you will be greeted with the info below.
Now, what if you have a user account in AD with the relevant rights, but no mailbox at all. In this case, you will be able to login to the ECP and administer your org depending on the rights assigned.
In short, companies who use a separate admin account in the 2003 world can continue doing it. Just add the relevant RBAC role membership and you will be able to administer using ECP. Hope this clears the confusion of many (who are posting questions in the forum) regarding whether a mailbox is required for an admin account to use ECP.