Exchange 2013 CAS Vs Exchange 2010 CAS

*** If I am not wrong In Exchange 2010 , any email you send or receive Internally  goes through CAS server, before it reaches the destination mailbox.  Also when receiving email from external users it should go through CAS before you receive it in you mailbox. Sending out email does not have to go through your internal CAS.
Well, am I correct on my above statement? correct me if I am wrong.

****Now for Exchange 2013. If an internal user sends an email to an internal user , does the email have to go through CAS(Front End Server) then to the mailbox server of another internal user ?

***I believe  if external user sends email to an internal user, it should come through CAS(Front End server), before it reaches internal user mailbox.

***the same as for OWA, if internal user wants to check their email using OWA , I believe the process of checking, should go through CAS(front end server) to determine where the user mailbox is, then redirect the user to their mailbox.

Any Exchange expert to clarify the confusion ?

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Will SzymkowskiSenior Solution ArchitectCommented:
in Exchange 2010 CAS provides availability to protocols to allow clients to connect to their mailboxes some of the protocol are https, imap, pop, rpc.

The role that actually routes mail is the Hub Transport role. When an email is sent from a mailbox the mailbox server that hosts the mailbox will place the message in the message submission queue and from there the transport role will pickup the message and route it to the appropriate connector based on the sending address and policies that are applied.

Internal messages are routed using the receive connector and external messages are routed using the send connector.

For Exchange 2013 it is different in that all of the "actual" routing is done on the server holding the mailbox server role.

CAS does have a Front End Transport but it really do not do any routing is simply passes the messages off to the mailbox servers and they do all of the routing from there.

When internal messages are send they do not go through the CAS mailbox servers will route message between themselves.

However when messages and sent or received from the internet they do in fact do through the CAS Front End Transport (this is somewhat transparent) because the front end transport is basically a pass through mechanism.

jskfanAuthor Commented:
In exchange 2010 , emails sent and received go through CAS server I believe .
Kamal KhaleefaInformation Security SpecialistCommented:
for internal messaging
in exchange 2013 the smtp msg dont go to cas transport service its just go across mailbox transport and move back to user mailbox without any communication with cas

but for relay you have to do it on cas server not mailbox if you do relay on mailbox role "as you may understand that internal msgs dont need cas" all your transport service will stop work

As relay still need to do authentication and thats only exist in cas service
The main difference between exchange 2010-2013 as following

Cas role do not search for user mailbox as b4
In 2013 the user mailbox name contain its address inside the database
Earlier in 2013 cas was doing query to find where this sam account mailbox holded "this make easy to migrate to newer veraions"
The other difference is database "replication log size" minimized from 5 mb to 1 mb to minimize the corruption effected data
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Kamal KhaleefaInformation Security SpecialistCommented:
Exchange 2010 and 2013 use cas to send emails
But in 2013 and only for mailboxes which exist internally it just go across mailbox transport service not cas
jskfanAuthor Commented:
<<in 2013 and only for mailboxes which exist internally it just go across mailbox transport service not cas>>

So for external emails(going out and coming in) will have to go through CAS Front end ?

what about just login through OWA, does CAS front end server authenticate users and redirect them to their mailbox ?
Kamal KhaleefaInformation Security SpecialistCommented:
For first two yes
Login owa and other service yes its through cas
As mailbox do not do any authentication process only cas responsible about it and it will redirect to mailbox
Will SzymkowskiSenior Solution ArchitectCommented:
In exchange 2010 , emails sent and received go through CAS server I believe .

CAS is used for availability and connectivity to mailboxes via different protocols. The Transport Role is what actually routes the mail where it needs to do, not CAS.

jskfanAuthor Commented:
Will Szymkowski

IN Exchange 2010 even when email is sent internally from one mailbox to another mailbox while both mailboxes are in the same server and in the same database they still have to go through CAS server, which is the one that locate  the destination mailbox.

In Exchange 2013. Mailbox server has both Transport Services as well as Mailbox Transport service, if I am not mistaken it also has Client Access Protocol (I could be wrong on this). So there should be a service or protocol that determines where the destination mailbox resides before the Transport Service or the MB Transport service routes the email to the destination mailbox.
Will SzymkowskiSenior Solution ArchitectCommented:
Why are you continually referencing the CAS SERVER!

there are 4 internal roles.

CAS - client access
Hub Transport  - routing and policies
Mailbox - mailbox databases
UM - unified messaging

Exchange 2010
When  an email is sent it does NOT go through CAS. Mailbox Role sends the email to the submission queue the Hub Transport Role categorizes the email and sends it to the appropriate connector the email if internal it is then send to the appropriate mailbox for delivery. If it is external then it will use either External DNS or a smart host to route the email on the internet.

I think what you might be referring to is "CAS persistent connection" to the mailbox. This does not mean the email goes through CAS. It means that the client needs to have a connection at "all times" to the mailbox database where the mailbox resides.

In Exchange 2013
The hub transport role is on the mailbox server routes all of the mail internally. If an email is sent out to an external source then the mailbox routes the email but it does still "pass through" the Front End Transport on the CAS server.

This is the same idea if a piece of email is coming from an External source it goes through Front End Transport first but "DOES NOT" route the message. It sends it to the exchange server that holds the mailbox role. The mailbox role has Back End Transport service which then categorizes the email and route it to the appropriate mailbox.

Kamal KhaleefaInformation Security SpecialistCommented:
For external emails
Mailbox server send it directly to internet without send it to cas first

If you want to send it to cas server you should edit send connector and check box proxy to cas server
Will SzymkowskiSenior Solution ArchitectCommented:
King2002..... Please take a look at the below article which outlines how mail routing works in Exchange 2013.

As you can see in the arcitecture diagram mail does flow through CAS for External Receive and External Send.

Although CAS does not do any of the "actual" routing it does go through the server transparently.


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jskfanAuthor Commented:
Thank you Guys!
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