MAIL
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. Mail1 and Mail3 are in a different Notes Named Network (NNN) than Mail2 and Mail4. Connection documents have also been created to route mail from Mail4 to Mail3, and Mail2 to Mail1.
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.
If different NNN are used and Connection documents are set up from Mail4 to Mail3 and Mail2 to Mail1, failover can be accomplished.
The backup Connection document from Mail4 to Mail1 should have a cost of plus 1 on the cost of the connection from Mail4 to Mail3. Therefore, Mail3 is used until it goes down, then it failsover to Mail1. The dynamic cost of Mail3 is increased by 1, which is equal in cost with Mail1, but lower Mail1 is used because it is lower alphabetically. The cost of the connection between Mail2 and Mail3 should equal the cost of the connection from Mail2 to Mail1. Mail1 is chosen when it is up because it is alphabetically lower than Mail3, but when Mail1 goes down, its cost is increased by 1, and it is higher than Mail3, so Mail3 becomes the failover route.
Configuring these servers requires: