From: Francis Dupont <Francis.Dupont@enst-bretagne.fr>
> => aliases (CNAME left parts) are legal in mail addresses.

But RFC 1123 says as below, for example...?  For more details, see
also

  http://www.exim.org/pipermail/exim-users/Week-of-Mon-20030203/049446.html

for instance.  In short, it does no "harm" in _most_ environments, but
affects many systems at least.  (I wonder what Dmitry meant by
"webmaster@caml.inria.fr is not routable"...)

----------------------------------------------------------------------
5.2.2  Canonicalization: RFC-821 Section 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
         CNAME.
----------------------------------------------------------------------

> => 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
SMTP, at least.  Of course, you could argue that bodies (From: and
To:) may be different from envelopes (MAIL and RCPT), though.

The bottom line is that it is still a bad practice to use aliases in
mail address domains for these reasons.  Instead, we can just use A
records in most cases.

> PS: alias is a DNS concept, SMTP requires valid names (aliases or canonical
> names) with a restriction in the character set (only digits, letters and
> the character "-" not in the first position) for labels (names are made up
> by labels separated by dots). Note that SMTP does not require the name
> exists...

Grrr...:-)

Best regards,

	Eijiro