Okay, this is tough for me. I'm semi-out of work right now (I'm doing some work here and there, enough to keep food on the table, which could be worse), so part of me is jealous.
The other part wonders how you got yourself in so far over your head.
That said, here is a pointer to some help to get you going. From the sound of your post, you should be prepared to learn a whole lot in a hurry, especially if your the one responsible for that companies system. Two sites you'll want to check out are:
The first is the qmail author's site, and that's often the best place to learn about a program. The second is a repository of documentation for linux in general. Most of it applies to all distributions, redhat, openna, or any other. Of course, check openna's site http://www.openna.com/
for things specific to openna.
Good luck, but remember, being a good sysadmin means reading a lot of stuff (often afterhours, on your own time), being careful to understand what you're changing before you change it, solid testing once you do change something, and most of all, keeping good records of what you change, so if you ever have a problem, you can go back and figure it out. Nothing worse than guessing why something that used to work doesn't anymore.
Again, good luck. I don't envy the amount of work ahead of you.