Here is a quick tip for Exchange Admins working in an Exchange 2010/2013 Coexistence Environment.
When you launch the Exchange Admin Center, https://mail.domain.com/ecp and you login, you will be presented with the Exchange 2010 ECP page and not your Exchange 2013 EAC page.
To get round this, you need to add a piece to the URL so it takes you there directly.
Instead of using the following:
You can now be sure that you will hit the Exchange 2013 EAC page and be able to administer your servers.
Hope it helps.