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Re: [ietf-smtp] SMTP server "RFC2821 Violation" for EHLO ip-literal.

2019-12-15 10:13:54
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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