Postfix smtp outgoing load balancing

I got good workaround for balancing smtp outgoing from postfix mailing list archives(not using expensive real load balancer). suppose we have 4 smtp servers for outgoing email, and we need to configure/load balance like this:

1                                                             +------------+
                                                             |            |
                        +----------> smtp backend a -------->|            |
                        |                                    |            |
    clients --------> smtp lb -----> smtp backend b -------->|  Intenet   |
                        |                                    |            |
                        +----------> smtp backend c -------->|            |
                                                             |            |
                                                             +------------+

Here’s the configuration :

Postfix section

main.cf (smtp lb/load balancer)

1/etc/postfix/main.cf:
relayhost = example.com

Here’s the configuration :

Dns sections.

named.conf

1/etc/named.conf:

options {
        directory "/var/named/data";
};

zone "example.com" in {
        type master;
        file "example.com";
};

example.com zone file

/var/named/data/example.com
$TTL 2D;

@ IN SOA a.ns.example.com hostmaster.example.com (
            1            ; Serial
            10800           ; Refresh after 3 hours
            3600            ; Retry after 1 hour
            604800          ; Expire after 1 week
            3600 )          ; Minimum TTL of 1 hour
@ IN NS a.ns
@ IN NS b.ns
a.ns IN A 192.0.2.1
b.ns IN A 192.0.2.2

@ IN MX    0 a.mx
@ IN MX    0 b.mx
@ IN MX    0 c.mx

a.mx IN A 192.0.2.3
b.mx IN A 192.0.2.4
c.mx IN A 192.0.2.5

By default relayhost(in main.cf) will listing all available mx on example.com and randomly shuffles mx with equal preference(Postfix client makes a random selection between equal-preference MX hosts), in this case these mx are our smtp backend for balancing outgoing mail.