I have two sites, A and B. In each site I have a mail server and DNS. On DNS I have two MX records : mail01.contoso.com MX 5 mail02.contoso.com MX 10
DNS zone are integrated in AD.
These two site are connected over VPN. I need to users from site A to send mail over mail01 and user from site B sent mail over mail02, and when one of mail servers down all user sent mail over second available mail server.