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Re: is not a "legal" mail address
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Date: -- (:)
From: Francis Dupont <Francis.Dupont@e...>
Subject: Re: is not a "legal" mail address
 In your previous mail you wrote:

   From: Francis Dupont <>
   > => aliases (CNAME left parts) are legal in mail addresses.
   But RFC 1123 says as below, for example...?

=> you mix up the MTA (which canonizes when it can *) and the MUA
(which should accept any valid name).
(* in RFC 821 section 3.7, not in RFC 2821 which was written after
a real DNS practice).
=> you still mix up MTAs and MUAs. In fact, you suggest *your* MTA is
wrong (:-)!

   (I wonder what Dmitry meant by
   " is not routable"...)
=> me too.

   5.2.2  Canonicalization: RFC-821 Section 3.1
=> note this is in section 3.7, not 3.1

            The domain names that a Sender-SMTP sends in MAIL and RCPT
            commands MUST have been  "canonicalized," i.e., they must be
            fully-qualified principal names or domain literals, not
            nicknames or domain abbreviations.  A canonicalized name either
            identifies a host directly or is an MX name; it cannot be a
   > => it rewrites it because it checkes it and gets the canonical name as
   > a side effect, not because aliases are not legal...
   As explained above, they were illegal in MAIL and RCPT commands of

=> but they are not more illegal. BTW the Sender-SMTP is likely a MTA.
Today the canonization is considered only as useful (if it is not performed,
the next MTA will have to resolve the alias again) but no more as mandatory
or necessary.

   SMTP, at least.  Of course, you could argue that bodies (From: and
   To:) may be different from envelopes (MAIL and RCPT), though.
=> I argue that your MUA should accept what you give as soon as it is
valid and it knows what to do with it.

   The bottom line is that it is still a bad practice to use aliases in
   mail address domains for these reasons.

=> not only it is not a bad practice (the first agent on the path can
resolve the alias, before RFC 2821 it was the first agent using SMTP at
its sending side) but it is a very common practice. What do you believe
aliases are for?

   Instead, we can just use A records in most cases.
=> I don't understand this comment about A RRs (do you suggest to use
only litterals?)

   > Note that SMTP does not require the name exists...
=> IMHO MX RRs are a nice idea. With HTTP 1.1 virtual hosting they are
more than necessary!


PS: for other readers: MTA = Mail Transfer Agent, MUA = Mail User Agent.
A MTA is the software which puts mails from node to node, a MUA is the
software which we use to read and write mails. SMTP (Simple Mail Transfer
Protocol) is the protocol used between MTAs even it may be used to submit
mails (i.e., from the MUA to the first MTA).