Exchange 2013 Client Access Server role act as a light weight Exchange Aware Proxy Server, which serves all the client (MAPI, EWS, OWA, EAS, POP, IMAP, EAC) request and proxy the requests to Mailbox server role. Exchange 2013 also works as a Layer 7 SMTP Proxy and router where it finds the best place to route the emails to the destination.
3 Main Components of Client Access Server Role
- Client Access Protocols
- SMTP
- UM Call Router
Client Access Protocols and the Client Connections
All the client protocols\connections like OWA, EMS, EAC, MAPI, EAS, POP/IMAP, SMTP, UM Calls will connect to the Client Access Server roles, respective request will be passed to the concerned component on the Mailbox Server role. For example, if a POP3 client connects to exchange 2013 mailbox, the connection will go to POP/IMAP component on the Client Access Server role and that component will talk to the POP/IMAP component on the Mailbox Server role
All the above client connections are received by the CAS server and proxy to the mailbox server role. If a UM call request comes to CAS, it will send an acknowledgement with the redirection and the request will be redirected to UM component on the Mailbox Server role.
Client Access Server as email router (Front End Transport Service)
Client Access Server role in Exchange 2013 has a component named Front End Transport Service, which will act as a SMTP proxy to handle all inbound and outbound external emails for the exchange organization. CAS SMTP component wont store any emails or email queue, it will receives the emails, inspect the emails and it will send the email to destination
High Availability for Client Access Server Role (CAS Array)
Group of Exchange 2013 Client Access Server roles in an Active Directory is function as a Client Access Array. The RPC Client Access Server Array model is removed in Exchange 2013, which means there is no RPC/TCP connection in Exchange 2013. It Supports only RPC/HTTP.
Outlook Connectivity
As mentioned above, Outlook Clients connecting to RPC end points in Exchange 2010 is not available in Exchange 2013. The RCP/TCP model is removed and all the outlook clients connect to CAS server by RPC over HTTP. With the RPC over HTTP connection, there is no need to specify the RPC CAS array name on the Mailbox Databases and the Pop up while activating the passive copy won't appear anymore for users
Intelligent Email Routing by CAS using Delivery Groups
Front End Service in Client Access Server role has two important components named SMTP Receive and SMTP Send.
SMTP Receive – It receives the emails from External SMTP and performs Connection filtering, Sender Filtering and Recipient Filtering by the Protocol Agents and the emails will be sent to Hub Selector component on the SMTP Receive Component. Hub Selector finds the where to send the emails and the email will be sent to SMTP Sent Component.
SMTP Send – It sends the email to SMTP component on the Mailbox Server role and if it external emails it will send the emails to smart host
Delivery Groups
Hub Selector uses the Delivery groups to route the emails. Delivery Groups find the best routing path to deliver the emails. Delivery Groups are not new in Exchange 2013, the Active Directory Sites, Mailbox Server and the DAG used when routing or to find where to deliver the emails are the Delivery Groups in Exchange 2013. Email delivery to which delivery group is selected based on the recipients mentioned on the email.
Ports used for Mail Flow in CAS
Port 25 is used to communicate between server to server and Port 587 is used for Server to Client Communication
If any of the above information is not clear, please leave your comments to edit the same.
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