How to correctly set DNS & hosts file for mail?


I know this question has been asked a couple of times already but I don’t seem to find a solution for me unfortunately. :frowning:

From my perspective everything should be OK and depending on which mail test tool I use some complain about something while others don’t…
Some mail server are unwilling to receive my mails due to Reverse DNS entries missing.

I have a server running on Oracle Cloud with HestiaCP 1.8.10 with exim4.
DNS entries are completely managed in the Oracle Cloud. So basically it doesn’t rally matter what’s setup in Hestias DNS entries. But I copied the entries configured by Hestia over to Oracle.

On Oracle cloud you have an internal IP and an external/public IP. This public IP is used by all of my domains.

Question here: Which entry in /etc/hosts is correct?
Should I set the internal IP to my domain or the public IP?
Currently it is set to the internel IP.

root@digioso:/etc# cat /etc/hosts localhost

::1 ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
ff02::3 ip6-allhosts
x.x.x.x digioso

Due to the hosts entry nslookup on prints the internal IP.
For my other domains it prints out the public IP. Makes sense, since there’s no host-entry for them.

I can imagine why the (reverse) DNS problems occur. If someone external looks up, they’ll receive the public IP. So there might be a mismatch of what the server sees compared to what everyone else sees.