finding out more about reverse DNS and fixing it?

[LRW]

We send out monthly e-mails to our customers, and I've just noticed we
get a few replies back from domains saying they're refusing e-mail
from ourserver because we don't have a reverse DNS.

Now, I don't know what that means. We lease our own server from
rackspace.com, but before I go to them I wanted to find out more about
the issue itself, and if there's anything I can do on my end before I
go to them with it.

I did a newsgroup search, but all I could find are posts WAY over my
head regarding RFC's and other things.

Can someone point me to something that explains what a no reverse DNS
error indicates and how to take care of it?

I'd appreciate it!
Thanks,
Liam