CAS Server + Exchange 2010

I am confused on the role of a CAS server.  Can someone explain it to me.
I've read everything and I still do not get it.
We have 2 HUBS, 4 MBX, 2 EDGE, 3 CAS

I need to know the email flow.

All protocols flow through the CAS.

If I send an email to myself, I know it goes goes thru the HUB then the mailbox server
Where does the CAS come into play.

OWA goes through a CAS then it hits your mailbox

If external mail come into our network, it goes through a smart host >  Edge > HT > Mbx > outlook
Looking at that flow I do not see where the CAS comes in.

I have read everything.
Can someone explain baiscally 2 concepts.

How the email message flows internally & External with the above setup?
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Em ManCommented:
Client Access Server provides connectivity for clients (eg Outlook, Outlook Web App, ActiveSync) to mailboxes.

in short, before it gets to your outlook client or retrieving the mails from the Mailbox Server the client connect to the CAS to get the emails.

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In simple terms, your outlook clients now connect to the cas server or casarray if you clustered them. Mobile phones using active sync and owa also use cas servers to render  your mailbox.

A cas is very important now.
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