On 12/15/19 6:42 AM, Alessandro Vesely wrote:
If we reject [A.B.C.D], why don't we also reject foo.example?
The reason why SMTP requires to accept such stuff, AFAIUI, is to allow a broken
server to still be able to do its job. Is it so? Compare that to, e.g., DANE,
where a certificate error can shut a server up.
IMO, hosts should not be required as a matter of SMTP protocol to use
DNS names, not even to send mail. DNS is simply a convenient way of
finding IP addresses and allowing applications to continue to operate
across infrequent changes in IP addresses. It is not "a control plane
for the Internet" (as some are now saying), it is not a call setup
protocol, and it is nowhere nearly universal.
There are very many IP networks that do not use DNS but which still use
SMTP to send mail, even if only to a forwarder. Before web browsers were
commonplace, many networks did not use DNS except to forward email to
the Internet. The popularity of web browsers has resulted in much
wider support for DNS, but there are still many networks that aren't
intended to support humans using web browsers. For some of those
networks, DNS is not a feature, it is a liability - it adds nothing of
value for them and is something that can break and cause (say) assembly
lines to fail.
Of course if an organization wants to refuse mail based on a MAIL FROM
address containing an IP address literal, it is free to do so. But
there is no particular reason I can see that an outgoing mail relay
needs to know its own DNS name, or even to have one.
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