To enable an Elastic Cloud Server (ECS) accessible to Internet users, you must register a domain name with a domain name registrar, configure a public zone in the DNS service, and change the registrar's NS addresses to those provided for you so that the DNS service can resolve the domain name.
Hosts in VPC subnets use private DNS servers provided by the DNS service to resolve private domain names.
PTR records are used to prove the credibility of IP addresses and domain names of email servers.
After a receiver email server receives an email, it checks whether the IP address and domain name of the sender are trustworthy, and further determines whether the email is spam. If the recipient fails to obtain the domain name mapped to the sender's IP address, it considers that the email is sent by a malicious host and rejects it. Therefore, it is necessary to add PTR records to map IP addresses of your email servers to domain names.