Now we go in the other direction. A query to our IP returns a domain name. For phone numbers, it would be called an inverse lookup.
We set up reverse DNS (rDNS) so that other mail servers do not classify us as spammers.
When an email goes out from our server to another mail server, it is very likely that the mail server checks by reverse DNS lookup if the sender IP corresponds with the sender hostname (sender IP means our server IP and the sender hostname is in the HELO, i.e. in the header data of an email). If there is no valid answer due to the missing reverse DNS entry, then this is not very trustworthy and our e-mail will be classified as spam. If this happens too often, in the worst case it can happen that our server IP is included in spam lists (DNS-based Blackhole List - DNSBL).
For the backward search we set a subdomain here, which we deposit later on our server with the mail software. The name is freely selectable, only this subdomain should not be used elsewhere. I decide for a relatively neutral subdomain srv1.linuxserversetup.com.
In the server settings under "Networking" we can edit the reverse DNS via the extended menu.
