OK, so I *think* it seems to work, at least for a domain - not sure how to test a single host yet - you probably have to specify it somewhere in the test mail.
So add your domain in the panel.
Now, from another box try doing the following:
telnet mx-backup.server.net 25
EHLO localhost.localdomain
MAIL FROM:<john@somedomain.com>
RCPT TO:<john@main-mx-server.com>
DATA
To:<john@main-mx-server.com>
From:<john@somedomain.com>
Subject:Test
Some body text
.
quit
Check /var/log/qpsmtpd/current to see it is received.
Then /var/log/qmail/current to see that it goes out.
That seemed to accept the mail without authentication, and then send it along. Need I to need see if it stores the mail.