We are in the process to migrate our current Exchange 2007 to Exchange 2013.
Our environment is as follows:
HT01= Hub CAS Server (Exchange 2007)
MB1, MB2 = Mailbox CCR Cluster (Exchange 2007)
CAS1= CAS Server (Exchange 2013)
MBNew1, MBNew2 = Mailbox Servers (Exchange 2013)
Emails for users that are on Exchange 2007 can send and receive external emails. But they cannot send emails to users that are on Exchange 2013.
Users on Exchange 2013 can send/receive externally, send to Exchange 2007 users, but cannot receive emails from Exchange 2007 Users.