[CentOS] Sendmail as a queue only server

[Mohd Ikhwan Bin Abdullah]

I would like to know is there anyone who know how to or a site with the how
to on configuring a sendmail box to queue the mail only when the primary
mail server is down. And when the primary mail server is up it will issue an
ETRN command to the queue server to start processing and pushing the mail
that is queued to the primary server.

I am very much aware about the MX record priority on the DNS server but just
need to configure the secondary mail server only.

Thank you.