How to use serialmail ?

Hello

I installed serialmail on qmail1.03 so that People in a domain could get
their mails even their local mailserver was down.

But I am a novice on serialmail or ETRN.

1 - Will I have to create same ones of their mailserver's mailboxes those
are available. ?
2 - Will I have to create same username and pass of  users on the big server
so that They can login and get their mails ?

That is
I have a domain called abc.com.
That domain works local mailserver. but its MX record is my big mail server.
They have a leasedline to reach the internet.
if the line is down, People in that domain want to get their mails.

How can I do this ?

Thanks