Ken, A little difficult to say what this without knowing what 13.67.59.89 actually is. If this is an Azure deployment, ReverseFqdn needs to be populated on the Public IP address resource. Please take a look here https://docs.microsoft.com/en-us/azure/dns/dns-reverse-dns-for-azure-service... Thanks, Christian -----Original Message----- From: NANOG <nanog-bounces@nanog.org> On Behalf Of Ken Chase Sent: Monday, February 26, 2018 1:31 PM To: nanog@nanog.org Subject: Re: MSFT reverse IP failure? Having a client doing a test from the MSFT exchange tools site, which is failing. NOQUEUE: reject: RCPT from unknown[13.67.59.89]: 450 4.7.1 Client host rejected: cannot find your reverse hostname, [13.67.59.89]; NetRange: 13.64.0.0 - 13.107.255.255 CIDR: 13.96.0.0/13, 13.104.0.0/14, 13.64.0.0/11 NetName: MSFT A bit of an oversight? /kc -- Ken Chase - math@sizone.org Guelph Canada