Client Access server can provide redirection when a user tried to connect a CAS server via Outlook Web App which is not located in its local Active Directory site. Clients that use Exchange ActiveSync, Exchange Web Services, POP3, and IMAP4 can't use redirection.
There is a sample given by Microsoft on TechNet.
"The mailbox of User 1 is located on Mailbox server 01. The mailbox of User 2 is located on Mailbox server 02, and the mailbox of User 3 is located on Mailbox server 03. User 1 can access their mailbox through Client Access server 01 without using redirection. If User 1 tries to access Client Access server 02, their browser will display a message that includes the correct URL for their Client Access server. The user will be prompted to click the Outlook Web App URL for their Client Access server. They won't redirected automatically. If User 3 tries to access Client Access server 02, that server will proxy their request to Client Access server 03. Client Access server 03 isn't Internet-facing, but can receive requests from other servers within the firewall. Proxying isn't visible to the user. "