Question : two sendmail  server run at the same time

I want to setup more than one linux sendmail server
to work as backup .
the two servers run at the same time to send and recevie
mail.
when the first mail server  is down , the the second
mail server will hold all the job ,and the mail stored
in the first mail server can also be synchronize with
the second mail server so all the mail can be received
by client even the first server is down.

HOw can I do this?

Answer : two sendmail  server run at the same time

Note.
If this is for relaying use MX records in DNS to control
the flow of mail

domain.com.     IN     MX 10 main.mxserver.domain.com.
          IN     MX 20 backup.mxserver.domain.com.

Then you have some mailhost where people pick up their mail
if that is not either of those servers they are set up to relay mail to that server for the domain.com but to use mx for rest of the world.  Using mailertable would work


domain.com     esmtp:[123.123.123.123]

where 123.123.123.123 is the ip address of the mailhost where popaccounts are kept.

The quiz is where do your users submit their mail?
Solution to that is to have


smtp.domain.com.     IN      CNAME main.mxserver.domain.com.
smtp.domain.com.     IN     CNAME backup.mxserver.domain.com.

This will do some load balanceing for submittance but
if one of them is down users will only be 50% successful in sending mail.

note bothe main.mxserver and backup.mxserver have to be A records