D. J. Bernstein
Internet mail
Internet mail infrastructure

IP addresses in mail

Bracketed IP addresses

A dotted-decimal IP address in brackets, such as [], identifies a domain owned by whoever owns the host at that IP address.

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.

Numeric MX records

Many clients, notably sendmail, support IP addresses in MX records:
     clueless.net. IN MX 0
Some clients will skip this MX record, since the name 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.