I am researching options at our disposal and the what could be best practice.

One option would be having SMTP routing between three CAS servers (2 in CA and 1 in AZ). Would this be just adding all three to the same CAS? Is there a possible issue having the third CAS server in another datacenter from the first two?

Another thought is routing SMTP traffic between the two CAS servers in CA and using the AZ server as a failover. If CA comes back online how would that impact AZ? What about a split brain scenario and how to avoid/prevent? 

Finally would it be best to leverage a third-party appliance to do this? I believe this can be handled out of the box.

Thoughts and ideas are welcome.

Than

CAS servers don't route that way.  Mailbox servers handle mail transport in Exchange 2013.

Exchange handles mail routing internally; there isn't anything to configure.

Probably all you need to do to Exchange is ensure that the DR site has inbound and outbound access to the Internet, create an outbound send connector from there, and an inbound receive connector.  In DNS, you'd add another MX record pointing to the DR site with a higher preference number so that it only collects mail when the primary site is down.  Note that it'll also collect spam because spammers often send to lower-preference servers thinking that they may be less well protected by antispam services.

I should have made this more clear, this is in a 2010 environment. The CAS servers are also hosting the Hub Transport role. 

You have posted in an Exchange 2013 forum.  The Exchange previous versions forums are here:  http://social.technet.microsoft.com/Forums/en-US/category/exchangeserverlegacy

The rest of my post still applies.

Hi,

Have you checked the suggestion Ed suggested? Is there any update with your issue?

Best regards,