smtpd: warning: hostname example.com does not resolve to address 203.0.113.1

One postfix instance at sub.example.com is pushing mail to example.com via LAN, which is both a mail server and a router:

Internet
|
|
203.0.113.1 
<example.com> (127.0.1.1 in /etc/hosts)
192.168.1.1
|
|
192.168.1.99
<sub.example.com>

Every time a LAN mail is received, a warning message is logged:

postfix/smtpd[1337]: warning: hostname example.com does not resolve to address 203.0.113.1

However, the domain does resolve to the address, with only one A record (but no rdns):

# dig example.com
example.com.          3600   IN   A    203.0.113.1
# dig -x 203.0.113.1
..                       1   IN   PTR  new-customer.isp.example

While reverse DNS is NOT correct, that is not what the warning says, nor does it explain why it is only triggered by LAN mail. As confirmed via tcpdump, the LAN mail server EHLOs with sub.example.com - which is known to the postfix server as 192.168.1.99.

What is triggering the warning?

Solution 1:

Solution:

smtp_host_lookup = native

Will make the client postfix instance behave the way other mail clients on that systems would - using /etc/hosts IPs aswell - after which the target system will no longer complain about seeing the client connect from an outside IP although knowing it as 192.168.x.x.