Stan is exactly right.  The following is also from RFC822 (sec 1.1 Scope).
We should remember where SMTP leaves off and vendor software implementations
pick up.  That point is the interface between SMTP and the local MTA
(message transfer agent) implementation.  In fact, this is the case with
almost all TCP/IP standards.  Rarely do they dictate a local implementation.
The confusion arises when we forget that.

Don

RFC822, sec 1.1 Scope, in part:
          Some message systems may  store  messages  in  formats  that
     differ  from the one specified in this standard.  This specifica-
     tion is intended strictly as a definition of what message content
     format is to be passed BETWEEN hosts.

     Note:  This standard is NOT intended to dictate the internal for-
            mats  used  by sites, the specific message system features
            that they are expected to support, or any of  the  charac-
            teristics  of  user interface programs that create or read
            messages.

-----Original Message-----
From: Stan Ryckman [mailto:[log in to unmask]]
Sent: Monday, September 30, 2002 2:38 PM
To: [log in to unmask]
Subject: Re: Dupe entries on 1.8d

......

Huh?  WHAT implementations ignore this?  Remember, the RFC says
the addresses JOE@... and joe@... are distinct, but it does NOT
specify whether they map into distinct mailboxes.  Newer systems
seem to map all cases into the same mailbox, but that's just
a (permitted) choice, ....