If email is a critical part of your organization’s day-to-day operations, redundant components, like load balancers, mail exchange (MX) records, and relays are necessary to ensure that the messaging system remains operational.
By using redundant MTAs, you ensure that if one component is disabled, the other is still available. Also, spreading resources across redundant MTAs enables load sharing. This redundancy also provides fault tolerance to the Messaging Server system. Each MTA relay should be able to perform the function of other MTA relays.
Installing redundant network connections to servers and MTAs also provides fault tolerance for network problems. The more critical your messaging deployment is to your organization, the more important it is for you to consider fault tolerance and redundancy.
MX records are a type of DNS record that maps one host name to another. Equal priority MX records route messages to redundant inbound MTAs. For example, the sending MTA from the Internet will find that the MX record for siroe.com corresponds to MTAA.siroe.com and MTAB.siroe.com. One of these MTAs is chosen at random, as they have equal priority, and an SMTP connection is opened. If the first MTA chosen does not respond, the mail goes to the other MTA. See the following MX record example:
siroe.com. in MX 10 MTAA.siroe.com siroe.com. in MX 10 MTAB.siroe.com
When Messaging Server hosts are each supporting many users, and there is a heavy load of sending SMTP mail, offload the routing task from the Messaging Server hosts by using separate inbound and outbound MTAs. You can further share the load by designating different MTAs to handle outgoing and incoming messages.
Often, both the inbound and outbound MTAs are combined as a single In/Out SMTP host. To determine if you need one or more MTA hosts, identify the inbound and outbound message traffic characteristics of the overall architecture.
Load balancing can be used to distribute the load across several servers so that no single server is overwhelmed. A load balancer takes requests from clients and redirects them to an available server by algorithms such as keeping track of each server’s CPU and memory usage. Load balancers are available as software that runs on a common server, as a pure external hardware solution, or as a combined hardware and software package.