How Mail from outlook sent to another outlook via Exchange server

The Mail Submissions Service on the Exchange 

2010 Mailbox Server notifies the Hub 

Transport Server that a message is in the Outbox. The 

Store Driver on the Hub Transport 

Server retrieves the message from the Mailbox Server. 

RPC is used for communication between the Hub and the 

Mailbox Server.


or 

1. The Microsoft Exchange Mailbox Submission service that's running on Mailbox1 notifies an Exchange 2010 Hub Transport server that's located in the same Active Directory site of the new mail item for transport.
2. Using RPC, the store driver component on an Exchange 2010 Hub Transport server in the same Active Directory site retrieves the message and puts it in the Submission queue on the local server.
3. From the Submission queue, the message moves through categorization. The categorizer first performs recipient resolution and determines that joe@contoso.com is an external recipient.
4. The routing component selects the best connector through which to route the message and calculates the least-cost routing path to that connector. In this example, a Send connector has the address space *.contoso.com and is the connector selected by the routing component. All the source servers for this Send connector are located in Site B.
5. The routing component determines the next hop required to reach a source server for the Send connector. The Hub Transport server in Site A queues the message for SMTP delivery to Site B.
6. If the receiving server in Site B is a source server for the Send connector, it queues the message for delivery to that Send connector. If the receiving server isn't a source server for the *.contoso.com Send connector, the message is relayed by using SMTP to a Hub Transport server in Site B that's the source server for the connector.

From Microsoft Site