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: warning: hostname router.intern does not resolve to address 220.127.116.11
(18.104.22.168 is a placeholder for its WAN IP.)
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.