From:
http://www.postfix.org/postconf.5.html#reject_unknown_reverse_client_hostname

reject_unknown_reverse_client_hostname
Reject the request when the client IP address has no address->name mapping. 
This is a weaker restriction than the reject_unknown_client_hostname feature,
which requires not only that the address->name and name->address mappings
exist, but also that the two mappings reproduce the client IP address. 
The unknown_client_reject_code parameter specifies the response code for
rejected requests (default: 450). The reply is always 450 in case the
address->name lookup failed due to a temporary problem. 
This feature is available in Postfix version 2.3 and later.

reject_unknown_client_hostname (with Postfix < 2.3: reject_unknown_client)
Reject the request when 1) the client IP address->name mapping fails, 2) the
name->address mapping fails, or 3) the name->address mapping does not match
the client IP address. 
This is a stronger restriction than the reject_unknown_reverse_client_hostname
feature, which triggers only under condition 1) above. 
The unknown_client_reject_code parameter specifies the response code for
rejected requests (default: 450). The reply is always 450 in case the
address->name or name->address lookup failed due to a temporary problem.

Jim B spoke forth with the blessed manuscript:
> How can postfix be configured so that postfix will do a reverse dns on incoming
> mail and if it fails then the mail will not be accepted or will be discarded?
> 
> 
> Jim
> 
> 
> 

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