In case your mail server is down, you can make 2 mail servers (Server A & Server B) ready and set 2 MX records in your DNS. Smaller number of the MX record priority (Server A) will receive the email first. If the Server A down, all the mail will go to Server B.
In case both mail servers are Qmail, and Server A is the destination. You do not need to set real mail box in Server B. But you have to set the config files in Server B:
In /var/qmail/control/smtproutes, add the following line:
domainname.com:address.to.which.the.mail.goes
So, in this case, 'address.to.which.the.mail.goes' should be the ip address of server A or the FQDN address of Server A.
For your reference, /var/qmail/control/locals is for domains that store the incoming mail in LOCAL mail box. If you have ONE domain on the olcal box, and 1 million being forwarded, it does not make much sense to add every single forward domain to the smtproutes file.
However, if you are more familiar with how vpopmail. Put eight lines in smtproutes that specify the domains, and the address to which they should be sent. Leave the nieth domain out, which means it will be stored locally.
