mail systems actually implement the parsing of the domain production. If you
give any number of popular, widely implemented
SMTP servers RCPT TO:<email@example.com>, will it query DNS for “184.108.40.206”,
or will it attempt to contact the server at
the IPv4 address 220.127.116.11 ? Implementers can answer this question...
Given that the chances of either being a useful address rounds to
zero, why is this question worth answering? RFC 5321 is quite clear
that this is not an address literal, since it's not in brackets, and
there will never be an all-numeric TLD in the public Internet.
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