Two-tiered Architecture—Messaging Data Flow

This section describes the message flow through the messaging system. How the message flow works depends upon the actual protocol and message path.

Sending Mail: Internal User to Another Internal User

Synopsis: Internal User > Load Balancer > MTA Outbound Relay 1 or 2 > MTA Inbound Relay 1 or 2 > Message Store 1 or 2

An increasingly more common scenario is to use LMTP to deliver mail directly from the outbound relay to the store. In a two-tiered deployment, you can make this choice.

Messages addressed from one internal user to another internal user (that is, users on the same email system) first go to a load balancer. The load balancer shields the email user from the underlying site architecture and helps provide a highly available email service. The load balancer sends the connection to either MTA Outbound Relay 1 or 2. The outbound relay reads the address and determines that the message is addressed to an internal user. The outbound relay sends the message to MTA Inbound Relay 1 or 2 (or directly to the appropriate message store if so configured). The MTA Inbound Relay delivers the message to the appropriate Message Store. The Message Store receives the message and delivers it to the mailbox.

Retrieving Mail: Internal User

Synopsis: Internal User > Load Balancer > MMP/Communications Express Proxy Server 1 or 2 > Message Store 1 or 2

Mail is retrieved by using either POP, HTTP, or IMAP. The user connection is received by the load balancer and forwarded to one of the MMP or Communications Express servers. The user then sends the login request to the access machine it is connected to. The access layer machine validates the login request and password, then sends the request over the same protocol designated by the user connection to the appropriate Message Store (1 or 2). The access layer machine then proxies for the rest of the connection between the client and servers.

Sending Mail: Internal User to an External (Internet) User

Synopsis: Internal User > Load Balancer > MTA Outbound Relay 1 or 2 > Internet

Messages addressed from an internal user to an external user (that is, users not on the same email system) go to a load balancer. The load balancer shields the email user from the underlying site architecture and helps provide a highly available email service. The load balancer sends the message to either MTA Outbound Relay 1 or 2. The outbound relay reads the address and determines that the message is addressed to an external user. The outbound relay sends the message to an MTA on the Internet.

Sending Mail: External (Internet) User to an Internal User

Synopsis: External User > MTA Inbound Relay 1 or 2 > Message Store 1 or 2

Messages addressed from an external user (from the Internet) to an internal user go to either MTA Inbound Relay 1 or 2 (a load balancer is not required). The inbound relay reads the address and determines that the message is addressed to an internal user. The inbound relay determines by using an LDAP lookup whether to send it to Message Store 1 or 2, and delivers accordingly. The appropriate Message Store receives the message and delivers it to the appropriate mailbox.