If you host your own email behind a NAT device or firewall, you’ve probably seen this issue come up at least once in your enterprise. Recently, I’ve received some complaints about exchange 2007 bouncing email messages with error messages like this: mxi5p.craigslist.org #554 5.7.1 <unknown[12.12.12.2]>: Client host rejected: rDNS/DNS_validation_failed._Please_setup_matching_DNS_and_rDNS_records: _http://www.craigslist.org/about/help/rdns_failure xabd-4 ## or its other… Continue reading Firewall, NAT and DNS, reverse DNS entries do not match Error