After installing Exchange 2013 for the first time I noticed some new changes in the architecture that made it confusing in regards to mail flow. One of the bigger changes in Exchange 2013 is the changes of Exchange server roles. In Exchange 2013 we now have just Mailbox and the CAS server role. There is no longer a standalone hub server role. The transport components that where traditionally on the hub role are now spread out on both the Exchange 2013 mailbox and CAS server roles. Let us first go ahead and talk about different transport services in Exchange 2013.
Next let’s talk about each of the ports mentioned above. Then finally we will discuss the flow of mail through this pipeline.
Here is a diagram that I put together to help me understand these 3 services and also what each of these ports are used for. Next we will discuss now mail flows through these services.
Now let’s talk about Exchange 2013 Mail Flow.
Sending Mail to both Internal\External recipients
Step 1: Mailbox Transport - This process starts with the user typing a message in Outlook\OWA and clicking the send button. In Exchange 2007\2010 it was the responsibility of the Store Driver in the Transport Service on the HT Server to send the message to the next hop. Now the Store Drive has been relocated to Mailbox Transport and split into two services (Submission and Delivery Services). The Mailbox Transport Submission service will pick up the message from the users Outbox, Run the Hub Selector process (in order to select the best Transport service which could be local or another server), and forward the message to the Default Receive connector in the Transport service (SMTP 25 or 2525).
Step 2: Transport Service - After the message has been accepted by the Transport service it will get put into the submissions queue. The submission queue will process the message and hand it off to the Categorizer which will do recipient resolution (expansion and bifurcation) and routing resolution. Next it will be place into the correct delivery queues. If the message is going to an external recipient it will use the correct send connector and either send directly to internet or proxy through the FET Service (Set-SendConnector <name> -FrontEndProxyEnabled $true). If the message targets an internal user the message will be send from the Transport Service to the Mailbox Transport Delivery service on the destination mailbox server. Once the Mailbox Transport Delivery service receives this message it will use local RPC to place the message in the users Inbox.
Receiving mail happens in the reverse order of Sending mail. Note that bifurcation always happens at the Transport Service level on the Mailbox Server. After bifurcation the message is sent via SMTP directly to the Mailbox Transport Delivery service over port 475 on the Mailbox Server where the mailbox of the recipient is currently mounted.
I hope this has been helpful in understanding how mail flow now happens in Exchange 2013. If you have any questions please feel free to contact me to discuss further.