According to RFC1123 /1124 all internet hosts should have both a forward and reverse DNS entry. Because the forward can be easily forged but the reverse is very hard to forge, making this simple check is something many mail servers (as well as other internet based resources) deploy to help protect themselves from spam and malicious use.
IP Address
Reverse DNS is one of the most basic internet requirements and its simply that the forward address (hostname to IP) should match the reverse address (IP to Hostname). To get this right isn't complicated and is best practice for all internet hosts and a requirement for most commercial mail server hosts.
If your having problems getting your zone file to resolve both ways, and you've checked the resolver for your IP block and that's setup correctly, then raise a ticket at the helpdesk, or make a post on the forum here and we'll do our best to help you out.
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