I am presented a mail server setup, that is working well. There's only a small issue, which makes me think, there's something wrong in the DNS setup. It is a small setup with a Debian mail server (postfix/dovecot) behind a router. The router has port forwarding configured and is named "router.intern" (IP 10.0.0.1). So incoming mail gets delivered to the mail server and outgoing mails are also fine. Every time a mail gets delivered, there is a reverse DNS check for the delivering server IP to block spammers. But there's also a reverse DNS check on the router's name:
postfix/smtps/smtpd[8088]: warning: hostname router.intern does not resolve to address 123.123.123.123
(123.123.123.123 is a placeholder for its WAN IP.)
/etc/resolv.conf:
domain intern
nameserver 10.0.0.1
Why does the mail server try do resolve the router's DNS name over WAN? I'm at loss where to search as everything else is working fine.