In Exchange 2016, you do not have all the roles like in Exchange 2010 and Exchange 2013. You only have 2 roles, the Mailbox Role and the Edge Transport role.
You might be wondering how mail flows within Exchange 2016. Let’s take a look:
The Mailbox server role has three transport services called the transport pipeline, take a look below at how mail flows:
Front End Transport service: Front End Transport service acts as a proxy for all incoming and outgoing emails. It does not perform any email queuing or an email inspection and does not communicate with mailbox transport service directly. It receives all external Mail traffic and then sends it to transport service.
Transport service (HT Service): This Transport service is similar to a Hub Transport server on Exchange 2010, which handles all SMTP mail flow and performs mail queuing, content inspection and categorization of emails. This service also executes an anti-spam and anti-malware inspection. It receives incoming email from the front end transport service and forwards it to the mailbox transport service.
**Very Important** The Transport Service never communicates directly with mailbox databases on your Exchange 2016 Server.
Mailbox Transport service: Mailbox Transport service is the service which sends incoming emails into their respective mailboxes. These are broken down into 2 more services:
- Mailbox Transport Submission service: – Mailbox Transport Submission service will retrieve message from the mailbox database using an Exchange Remote Procedure Call (Short for RPC) and then delivers it to transport service in order to send emails to respective recipients.
- Mailbox Transport Delivery service: – Mailbox Transport Delivery service accepts emails from the transport service and connects to mailbox database using the Remote Procedure Call and stores them in the respective mailbox databases.
A Final Note: The Mailbox Transport service does not connect with Front End Transport service or the Mailbox Transport service or the mailbox databases on any other Mailbox servers.
Hope it helps.