For anyone interested, I manage my own DNS but of course my ISP's manage reverse DNS. I use Allegiance Cable and SWBell(AT&T) and to my surprise, both were more than willing to reverse DNS to my actual FQDN's. This makes proper setup of your Zimbra install right on the money.
I think it depends on what type of account you have. If you have a consumer level dsl/cable account it's doubtful your ISP will change the reverse lookup for you. I have a business level account with Comcast, and they set my reverse DNS with one easy call. I was actually shocked how easy it was. One of my friends has a business level DSL account with AT&T and they changed his reverse as well.
Unless you have a decent ISP that allows you to manage your own reverse DNS, mine does. :D In general you're right, some ISPs will do it for you and others won't.
Originally Posted by bradb21