How can we change the default behaviour of mailbox server routing outbound emails to proxy the delivery through a CAS?
In Exchange 2013, mailbox servers route outbound emails through a send connector by default. This behaviour can easily be changed and the delivery can be proxied through CAS if you so wish.
In order to configure the proxying, launch EAC and navigate to Mail Flow –> Send Connectors and edit the send connector which routes emails to the internet.
Check the box that says Proxy through client access server and click Save.
Sure enough, outbound delivery was proxied through the CAS ;)