Exchange 2013 - Proxy through client access server not working

Hello All -

I recently migrated our company to Exchange 2013 and noticed that our email was leaving through the mailbox server. I put a check mark in the Send Connector where it says "Proxy through Client Access Server" and my mail is still coming from the mailbox server. How can I go about fixing this problem?

Environment:

1 CAS Server
1 Mailbox Server

Both server are behind the firewall with only port 25 opened to the CAS. The CAS has a Send Connector to a smart host for all messages.

Thank you!

Ryan