Inbound mail flows through an Exchange Server deployment in the following manner:
The sending SMTP server queries Domain Name System (DNS) to locate the mail exchanger (MX) resource record of the recipient's SMTP mail server. This MX record resolves to a corresponding host (A) record that resolves the IP address of the recipient's SMTP mail server.
The sending SMTP server initiates a conversation on the recipient's SMTP server (using port 25). On an Exchange Server gateway, the recipient's SMTP server is the SMTP virtual server on the Exchange server that is configured to accept inbound mail.
If the message is destined for a recipient of its SMTP mail domain, the SMTP server accepts the inbound message, as defined by recipient policies. For more information about defining recipient policies, see the following Microsoft Knowledge Base articles:
260973: Setting Up SMTP Domains for Inbound and Relay E-mail in Exchange 2000 Server and Exchange Server 2003
326139: White Paper - Exchange 2000 Recipient Management
When the message is accepted, the message is persisted in the \Queue folder on the Exchange server. The SMTP virtual server submits the message to the Advanced Queuing Engine, which then submits the message to the message categorizer. For more information about the Advanced Queuing Engine, see the following Microsoft Knowledge Base articles:
260973: Setting Up SMTP Domains for Inbound and Relay E-Mail in Exchange 2000 Server and Exchange Server 2003
233358: Message Handling for an Inbound SMTP Message Meant for the Local Store
The message categorizer validates the recipients of the message, checks for proper recipient attributes, applies limits and restrictions, flags the message for local delivery, and then returns the message to the Advanced Queuing Engine.
The Advanced Queuing Engine submits the message to the Local Delivery queue.
The Exchange store receives the message from the Local Delivery queue.
Mail messages are delivered to the client (for example Outlook, Outlook Express, or Outlook Web Access).