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Date: Fri, 27 Aug 2010 16:01:44 -0700 (PDT)
From: Don Lewis <truckman@FreeBSD.org>
To: ronald-freebsd8@thuis.klop.ws
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Cc: freebsd-stable@FreeBSD.org, avg@icyb.net.ua
Subject: Re: cname replace in mail address? [off-topic] (Re: Attn Ronald
	Klop)
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On 27 Aug, Ronald Klop wrote:
> Mandatory? I'm googling, but can't find a document that declares it  
> mandatory and only sendmail seems to do it.
> I think it is lame to use DNS info to rewrite e-mail addresses, but the  
> person who made it 'mandatory' will have good reasons for it.
> 
> Does somebody have a pointer to the specs about this?

<http://www.rfc-editor.org/rfc/rfc1123.txt>

      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.