A PTR record is a type of DNS record that maps an IP address to a domain name. Reverse lookup checks are used to verify that an IP address is associated with a valid domain name, and are commonly used in anti-spam and anti-phishing measures. In the case of an external mail server, a PTR record would need to be configured in the DNS server for the domain name that corresponds to the server's IP address. This would allow reverse lookup checks to verify that the mail server is authorized to send email from the domain name associated with its IP address.