In this example, two IBM® Lotus® Domino™ servers, Mail1 and Mail3, route messages from the Acme organization destined for other Internet domains (external addresses) and receive mail addressed to the Acme Internet domain (acme.com). Mail1 and Mail3 have the field "SMTP used when sending messages outside of the local Internet domain" enabled on the Router/SMTP-Basics tab of the Configuration Settings document that applies to the servers and have the SMTP listener task enabled on the Basics tab of their Server documents.

Mail2 outbound relays to Mail1 and Mail4 outbound relays to Mail3.

If a user on the Acme internal mail server Mail2 sends a message to an external address -- one with a domain other than acme.com -- the server routes the message to Mail1, which can route mail to external domains. If a user on the Acme internal mail server Mail4 sends a message to an external address -- one with a domain other than acme.com -- the server routes the message to Mail3, which can route mail to external domains. This splits the load of outbound messages -- half route to Mail1 and half route to Mail3.

Any mail from an external Internet domain -- one other than acme.com -- is routed to either Mail1 or Mail3. The external DNS has two MX records for the acme.com domain, one for Mail1 and one for Mail3. When an Internet mail server tries to connect to the acme.com domain to transfer a message, it looks up acme.com in the DNS. The server finds the MX records for acme.com and, based on the record preferences of the MX records, returns the IP address of either Mail1 or Mail3. If the MX records have equal weight, the server randomly selects one of the records and returns the IP address of that record's server. Should that server be unavailable, the other MX record is selected and the IP address of the other server is returned. This provides load balancing through the random selection of the MX records when record preferences are equal and provides failover since the DNS shifts to another MX record when a connection fails. Once the mail reaches Mail1 or Mail3, that server routes the message to its destination.

The internal mail servers can route Internet mail to the server with SMTP enabled for external mail either via IBM® Lotus® Notes® routing, with a Foreign SMTP Domain document and SMTP Connection document linking to the SMTP server, or via SMTP routing, with the SMTP server configured as the relay host.

Configuring these servers requires:

• Enabling "SMTP used when sending messages outside of the local Internet domain" for Mail1 and Mail3.
• Enabling the SMTP listener task for Mail1 and Mail3.
• Setting up DNS correctly to include MX records for Mail1 and Mail3, indicating to external SMTP systems that these are the hosts that receive inbound mail for the acme.com domain.
• Either enabling "SMTP allowed outside of the local Internet domain" for the internal mail servers, Mail2 and Mail4, and listing Mail1 or Mail3 as the relay host, or creating a Foreign SMTP Domain document and SMTP Connection document that define the route to Mail1 or Mail3.

Sample mail routing configurations

Glossary