Help -> Setup MX backup serve!

[Renato Forti]

Hi All,
I am tring setup one MX backup server! And I need help!
For sample, I have 2 MX:
10 mail.mydomain.net
20 mail.backup.mydomain.net
In my main server (10) I have this on main.cf
myhostname = mail.mydomain.net
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
myorigin = /etc/mailname
mydestination = $myhostname, localhost.$mydomain, $mydomain, mail.$mydomain, www.$mydomain

How I should configure Backup MX serve. I did try this, but don’t work!

In my backup server (20) I have this on main.cf

myhostname = mail.backup.dokmail.net
mydomain = mydomain.net
myorigin = $mydomain
inet_interfaces = all
mydestination = mail.backup.mydomain.net
relay_domains = mydomain.net, $mydestination
relay_recipient_maps =
queue_run_delay = 200s
content_filter = smtp-amavis:[127.0.0.1]:10026
smtpd_recipient_restrictions = current_restrictions, permit_mx_backup
myhostname=mail.backup.mydomain.net
relay_domains= mydomain.net, $mydestination
transport_maps=mydomain.net smtp:mail.mydomain.net:25

What is wrong? How I can fix? How I can see messages in backup server? and test if it is working and relay messages to primary (when it up again!)?
Thanks for help!