Why does the MXToolBox checker report a warning for MX records hosted in Plesk: Reverse DNS is not a valid hostname?

Question

Why does the MXToolBox checker display this warning about an MX record hosted on the Plesk server?

PLESK_WARN: Reverse DNS is not a valid hostname

Answer

This warning message is displayed because the server's hostname is a second level domain name (e.g example.com), but according to the RFC guidelines, the hostname should be a third level domain name (e.g. subdomain.example.com).

Second level domains can very rarely be marked as an invalid hostname, thus failing rDNS checks. Mail servers may mark the Plesk server's outgoing messages as spam because of this.