A PTR (Pointer) record is a DNS (Domain Name System) record that maps an IP address to a domain name. It is the reverse of an A record, which maps a domain name to an IP address. PTR records enable reverse DNS lookups. Required for SPF, DKIM, and DMARC to verify mail servers and prevent spam.
Many mail providers (Gmail, Outlook) reject emails from servers without a valid PTR record. Learn more: https://en.wikipedia.org/wiki/PTR_record