Sharing in brief, Mail routing Concepts of Exchange server 2013, it has changed a lot from previous version of Exchange. Microsoft has reduced the number of roles to two i.e Mailbox server & Client Access Server.
There are few services that run on CAS & few other that applies to Mailbox Role, both of them combined together for successful mail delivery.
Below are the 4 services that combines together for mail routing:
- MSExchangeFrontEndTransport --> This is front end Transport service which runs on all CAS servers. This service is stateless proxy & is used for incoming and outgoing external email.
- MSExchangeTransport -->This service acts as middle-ware between Mailbox Transport service & Front-end transport service. All SMTP mail flow, message categorization and content inspection is also associated with this Service. It runs on all mailbox servers. This service connects Mailbox transport service on Local or remote servers for message delivery..
- Mailbox Transport service --> This Consists of two services MSExchangeDelivery & MSExchangeSubmission. (This runs on all mailbox servers)
MSExchangeDelivery :- This service receives SMTP messages from Transport service & delivers to exchange using RPC.
MSExchangeSubmission:- It retrieves messages from the Mailbox database using RPC & submits them to transport service via SMTP. It runs the Hub selector process to determine the best Transport service Local or remote.
Its important to note here : Transport service routes the message to Mailbox transport service of the DAG that holds the active database Copy. DAG can span multiple Ad sites & acts as a routing boundary instead of Active Directory.(which is used for mailbox servers that are not part of DAG)
Here is the Sample email flow between servers:
Mailbox Database (SRV1) ----->Mailbox Transport Service ----> SMTP to transport service on other Mailbox servers ----> Transport Service ------> Mailbox Transport service ------> Mailbox Database (SRV2)
Here is the Visual Depiction from Microsoft:
Mailbox Transport service makes the decision of next Hop based on Number & recipient type.
- For single Mailbox recipient, Message is routed to Target Mailbox sever within the proximity of Active Directory Site.
- For multiple mailbox recipients, use the first 20 recipients to select a Mailbox server in the closest delivery group, based on the proximity of the Active Directory site.
- If the message has no mailbox recipients for example mail user , select a Mailbox server in the local delivery group.