How do I set up reverse DNS?

Hi, i would like to know how should be set up our reverse DNS. We have 2 different servers.
1 is netax.cz and 1 is ntbox-01.netax.cz. And we host a couple of domains. When I diagnose for example mail.netax.com - it points to netax.cz and everything is OK. But when I diagnose for example mail.ntbox.cz, this goes to ntbox-01.netax.cz and there are couple of issues -

1. May be an open relay.
2. Reverse DNS FAILED! This is a problem.
3. Warning - Reverse DNS does not match SMTP Banner

Please how can I solve all these issues? Do I have to set up only the ntbox-01.netax.cz or do I have to setup all the other domains which we host at ntbox-01.netax.cz?

Once reverse DNS will be ok for the ntbox-01.netax.cz, will it be ok also for all the domains that have hosting at it?

Thank you..