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Because understanding the e-mail message flow is important, I will list some high level steps in the message flow:

1.  MAPI client sends a message to a remote recipient
2. Information Store (Store.exe) receives the message
3. The created MailMsg object is forwarded to the Advanced Queue Engine (AQE)
4. The Message Categorizer from the AQE processes the MailMsg object and splits it into MIME or RTF as necessary
5. The Message Categorizer expands groups and checks defined Message limits on Exchange
6. The MailMsg object is then transferred to the Remote Destination Domain within the AQE
7. The AQE passes the destination address to the Exchange Routing Engine
8. SMTP initiates an SMTP session with the remote SMTP host
9. After the SMTP session with the remote host has been established, the information store retrieves the body of the message and converts the message as necessary
10. SMTP sends the Message from the Queue to the Remote Host

Exchange InterProcess Communication (EXIPC)

EXIPC is responsible for Data Transfer between Internet Information Server 6.0 / 7 (IIS) and the Microsoft Exchange Server Information Store (MSExchangeIS). EXIPC provides a layered service between both components to achieve the best possible performance between IIS dependant components and the Exchange databases. As you might know, all Internet Client Access Protocols like HTTP/S, SMTP, POP3 and IMAP4 are configured and managed by IIS with some exceptions.

Information Store (Store.exe)

The Microsoft Exchange Server Information Store (Store.exe) is the end point for e-mails sent to users on this server.　It is also　the start point for e-mails which are sent by MAPI clients, like Microsoft Outlook 2003, which directly connect to the MSExchangeIS.

This is a living document and may change over time.

Enjoy !!!