Exchange 2013 comprises of three transport services:
- Front-end transport service: Situated on the CAS it simply acts as a stateless proxy for outgoing internal and external SMTP traffic.
- Transport Service: Situated on the mailbox server - it is almost identical to the Exchange Server 2010 role - Hub Transport - simply providing mail routing accross your organization. (e.g. to different mailbox servers - although this role does NOT interact with any mailboxes directly!)
- Mailbox Transport Service: Situated on the mailbox server - it comprises of two services: Mailbox Transport Submission and the Mailbox Transport Delivery service. The Mailbox Transport Delivery service recieves mail from Transport service on the local server (or a server in the same site) and delivers the mail directly to a mailbox via RPC, while the The Mailbox Transport Submission service submits mail to the relevent Transport Service.
Below is a summary of a mail item being recieved from the internet destined for an exchange mailbox:
1. The external mail server sending the mail looks up the MX records for the destination domain and then routes it to the appropriate mail server.
2. The mail should then be recieved by a recieve connector setup in your Exchange environment on the CAS.
3. The mail is picked up by the front-end transport service and then forwarded to the transport service on the mailbox server.
4. The Transport service on Mailbox Server then categorizes the email, performs message content inspection, etc. Since it doesn’t connects directly to Mailbox database it sends email to the Mailbox Transport Service over port 25.
5. The Mailbox Transport Service is again divided into two service out of which Mailbox Transport Delivery service receives SMTP message from Transport service
6. The Mailbox Transport Delivery Service using Store Driver would connect to the mailbox database via RPC and deliver the e-mail to the mailbox database
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