Tried various possibilities to configure the secondary MX server to queue mails properly if primary MX fails. Today tried the secondary MX setup with below configuration and it worked out.....
In main.cf file,
relay_domains = $mydestination example.com
smtpd_recipient_restrictions = permit_mynetworks check_relay_domains
Included the transport_maps too in main.cf file in which configured the domain smtp transport address: transport_maps = hash:/etc/postfix/transport
In /etc/postfix/transport file,
If primary mail server for the domain example.com will down then all the mails are backuped/queued
in secondary/backup mail server[postfix] once the primary server will be ready then mails are delivered
from backup to primary server.