How to configure load balancing for MX records with 2 mailrelay servers?

Since your are posting in DNS, I assume you are talking about DNS based load balancing.
http://www.zytrax.com/books/dns/ch9/rr.html#mail

An alternative would be a gateway(firewall) load balancer. But this requires that you provide information about your environment.

I have configured my DNS as below:

        IN  MX  10  mail.example.com.
IN  MX  10  mail.example.com.

....
mail    IN  A       192.168.0.4
        IN  A       192.168.0.5
        IN  A       192.168.0.6

We have 2 Mailrelay servers and the given MX records are pointed to these mailrealy servers respectively. I have configured my DNS as below:

IN  MX  10  mail.example.com.
IN  MX  10  mail2.example.com.

IN  A       192.168.0.4
IN  A       192.168.0.5

In this scenario how DNS round robin Works? Will it provide a failover solution? What will happen If  the mail.example.com. fails?

IN  MX  10  mail.example.com.
mail    IN  A       192.168.0.4
        IN  A       192.168.0.5
        IN  A       192.168.0.6

In this scenario - it's up to BIND which ip to supply. So the remote mail server will get only one of those IP and if it's unreachable (relay server down), the delivery will fail.

If you specify
   IN  MX  10  mail.example.com.
   IN  MX  10  mail1.example.com.
   IN  MX  10  mail2.example.com.
mail  IN  A       192.168.0.4
mail1 IN  A       192.168.0.5
mail2 IN  A       192.168.0.6

It's up to remote mail server to decide. it will see all of the possible IP and since it its logic incorporates it, it will resend using other MX records.

I have the 2 MX record with same priority. If  one of the MX record (mail.example.com.) is not reachable then will the emails sent to me Bounce or will it be delivered using the second MX record (mail2.example.com) automatically.