Exchange 2013 certificate issue

I have Exchange 2010 and 2013 installed in my environment. My goal is to migrate all the users from 2010 to 2013. I am testing internal Outlook connectivity and I get the following error when connecting to a mailbox on the 2013 server:

I need my CAS server to present the certificate name as mail.domain.co.uk - how can I do this?

Thanks

Dave