I need to know if the mail sent between mailboxes on the same MBX server goes through CAS (outlook or OWA or activesync) or just in the MBX

Also if the mailboxes are in the same org but different MBX servers

Thanks

To answer your question, I'll answer it in several parts.

In order to send/receive an email, you use a client (Outlook, OWA, ActiveSync) which only communicates with the CAS server.

Internal emails stay within the mailbox servers, whether it's going from one DB to another on the same MB server, or whether it's another MB server using the hub transport services.

Now, if we're talking about external email coming in or out, then it traverses the CAS server, on it's way to or from the MB server.

The following link has a picture (and verbiage) that explains the process. I believe the image is from MS.

http://exchangeserverpro.com/exchange-2013-mail-flow/