> On Feb 4, 2018, at 5:46 PM, J Doe <gene...@nativemethods.com> wrote: > > Feb 4 15:05:46 server postfix/smptd[718]: warning: hostname > 1-2-3-4.dyn.isp.net does not resolve to address 1.2.3.4: Name or service not > known > > Does this mean that: > > 1. smtpd receives a connection from an smtp client and does a reverse DNS > lookup > 2. smtpd performs a forward DNS lookup on the result and compares the > resulting IP address to the initial IP > 3. If the IP addresses don’t match it reports this error > > ... or is some other logic used to generate the error message?
The message happens when the hostname obtained from 1 fails to resolve to an IP address that can be compared in 2. The error is a hard error (NXDomain). -- Viktor.