A reverse DNS (configured as a PTR record) is the association of a domain name with a given IP address. The standards of reverse DNS/PTR mechanism are described in the following RFCs: RFC1033 and RFC1912 Section 2.1
The best practices for servers is to have a PTR associate with each IP addresse used to send emails. The domain provided in the reverse DNS belongs to the responsible parties and usually have a web site set up with the appropriate contact information for the domain.
**The domain provided in the reverse DNS needs to be a valid a record (domain or sub-domain) that resolve to the server IP address.**
If your server has more than one IP address, please ensure that all IP addresses have a corresponding PTR record (not just the first one). It is also recommended to set up that domain for your SMTP service banner, to keep consistency.
Example of a reverse DNS configuration:
mail1.client_domain.com or server1.client_domain.com
How to verify the PTR record of your server IP:
You can use the command “host”. For example if your IP is ww.xx.yy.zz:
# host ww.xx.yy.zz
zz.yy.xx.ww.in-addr.arpa domain name pointer
You can also use online tools such as: