Thursday 20 June 2013

How email flows in Exchange 2010

In Exchange 2010, all messages are always routed through the Hub Transport Server.

Mail Submission Service – when a message is created and the Send button is clicked, the new message is placed in the mailbox outbox. There’s a service running on the Mailbox Server role called the “Exchange Mail Submission Service” which notifies the Hub Transport Server that a new message is awaiting for processing. The Mailbox Server has an internal list of Hub Transport Servers in the same Active Directory site (the submission server list) which is updated every 10 minutes. This is done by the server discovery process. A round robin mechanism is responsible for load balancing the SMTP traffic across these Hub Transport Servers

Store Drivers – the Hub Transport Server’s Store Driver retrieves the message from the Outbox and puts it in the Submission Queue on the Hub Transport Server. The Store Driver uses RPC to retrieve the message from the Mailbox Server. There’s no traffic on port 25 (i.e. SMTP) between the Hub Transport Server and the Mailbox Server.

Submission Queue – this is a queue, located on the Hub Transport Server where all messages are stored that need to be processed. Not only the Store drivers can store messages in the submission queue, but this can also be done through a receive connector or the pickup directory.

Categorizer – the categorizer retrieves messages from the submission queue and determines where the message needs to be sent to. This can be an internal Active Directory recipient or an external recipient. The categorizer also expands distribution groups and identifies alternative recipients or forwarding addresses.

Pickup Directory – this is a directory that is checked once every 5 seconds for new messages. When a message is in the correct EML format it is picked up from this directory and when the process is completed the file is deleted from the pickup directory.

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