Does anyone have an example for the format we need to use in the DNS record generator? I'd like to have it generate a bunch of w.x.y.z.mydomain.com records so that each of my clients has a valid DNS and reverse dns entry.

I've looked for examples, but failed so far.

I've been told by our ISP that any one IP can only have one reverse record which means that only one domain can have a RDNS record.

I don't know this to be true for sure but it kind of makes sense unless somehow the reverse lookup returned all the possible domains at the IP.

If you think about it, it's pretty obvious that any given IP can only have one reverse record.

What's the reverse of 192.168.1.1? example.com. Not what's the reverse of 192.168.1.1? Still example.com. It's never gonna change because the query is always "what's the reverse of 192.168.1.1?"

The good news is that reverse records don't matter. You just need to have one for each IP that you intend to send mail from. It doesn't need to match any of your forward records (so virtualmin.com can, and does, have a reverse of ee.4.5646.static.theplanet.com and it works fine), it just needs to exist.

But, of course, if you actually have one IP per customer, you could do that...but you're not going to be able to convince Postfix to send out on a different IP address based on the sending user (and there's no reason to do so).

Record Generators are discussed here in the Webmin documentation:

http://doxfer.com/Webmin/BINDDNSServer#Setting_up_partial_reverse_deleg