From: John C Klensin
Date: 2020-07-12 11:42
To: John R Levine; ietf-smtp
CC: YAO Jiankang
Subject: Re: [ietf-smtp] Are A-label and U-label addresses supposed to be
(adding Jiankang since I'm not sure he is on this list)
Thanks for adding me. I will add more comments after yours.
--On Saturday, July 11, 2020 22:30 -0400 John R Levine
Let's say I had these two addresses, which are the same except
that one has a U-label and the other has the equivalent
A-label. They're both non-ASCII addresses since the mailbox
name is in Chinese
Presumably if you're sending mail to those addresses, you
should send them to the same place.
Given the way the interactions between RFC 6531, RFC 5321, and
IDNA work, you have little choice: the sending MTA needs to go
through IDNA to get an FQDN that can actually get looked up and,
after that, the place where it sends (or tries to send) the
message is all about the MX records.
But on the final delivery
MTA, is that one address or two? Different mail software
implement it differently.
I think that it SHOULD implement it in the same way. That is to treat these two
addresses to be same.
Besides john's example,
there is another example:
Do we regard these two addreses to be same?
I think that it is yes.
user(_at_)EXAMPLE(_dot_)com is another form of user(_at_)example(_dot_)com.
For the same reason,
用户1@后缀.services.net in another form of
Looking at RFCs 6530 and 6531 I get the impression the authors
assumed they'd be the same but I don't see anywhere it
explicitly says so.
Speaking as one of the authors, I don't think we discussed it
(rather than assuming anything). There is a specific reason why
we probably didn't, which goes to the "what questions are you
really asking" issue. The core SMTPUTF8 specs rather strongly
discourage using anything but native character UTF-8 strings
anywhere other than in the SMTP client when it is trying to
figure the next hop out.
Besides john's explanation,
RFC6531 has some clarification.
Section 3.7. Additional ESMTP Changes and Clarifications
The information carried in the mail transport process involves
addresses ("mailboxes") and domain names in various contexts in
addition to the MAIL and RCPT commands and extended alternatives to
them. In general, the rule is that, when RFC 5321 specifies a
mailbox, this SMTP extension requires UTF-8 form to be used for the
entire string. When RFC 5321 specifies a domain name, the
internationalized domain name SHOULD be in U-label form if the
SMTPUTF8 extension is supported; otherwise, it SHOULD be in A-label
So in the email address, both U-label and A-label are different forms for the
Internationalized Domain Names.
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