RFC 1123 requires, for unspecified reasons, that each host ``accept and recognize'' a domain literal for any of its own IP addresses. Many SMTP servers automatically convert their own IP addresses into particular local domains. However, there are many domains that cannot be reached this way.
clueless.net. IN MX 0 188.8.131.52.Some clients will skip this MX record, since the name 184.108.40.206 does not exist in DNS. Sites that set up numeric MX records end up losing mail. However, there are thousands of numeric MX records, so I strongly encourage clients to support them.