About the PTR / Reverse DNS
A PTR record maps an IP address back to a host name. Mail servers are expected to have a PTR whose host name resolves back to the same IP — called forward-confirmed reverse DNS. Many receivers reject or penalise mail from IPs without it.
PTR records are managed by whoever owns the IP address — your hosting or VPS provider — not in your domain's DNS. Ask them to set it to your mail host name, such as mail.yourdomain.
What is reverse DNS?
A PTR record maps an IP address back to a host name. Mail servers should have a PTR that resolves back to the same IP (FCrDNS). It is set by whoever owns the IP — usually your hosting provider.