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2001 Dec 30 16:59

The following is an archive of a post made to our 'vox-tech mailing list' by one of its subscribers.

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Re: [vox-tech] E-mail recipient?
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Re: [vox-tech] E-mail recipient?


  • Subject: Re: [vox-tech] E-mail recipient?
  • From: Matthew Roper <mdropeMAPSr@ucdavis.edu>
  • Date: Sat, 16 Dec 2000 21:51:06 -0800
  • References: Pine.GSO.4.21.0012162045001.17397-100000@runner.ucdavis.edu

I believe that there are actually two sets of headers attached to every
email: one set for the "envelope" and one contained inside the email
itself.  The envelope helps the email travel across the Internet, but gets
discarded when it reaches its final destination.  The header inside the
email itself does absolutely nothing other than get displayed by email
clients.  If you use sendmail or a mail client to send an email, the
program will only take one set of To: and From: headers and use those for
both the envelope and the internal message headers.  However if you talk
to the SMTP server directly, you can use different values.  Here's an
example (you can test it out yourself by telneting to port 25 on a local
SMTP server)::

Server:
220 localhost.localdomain ESMTP Sendmail 8.9.3/8.9.3; Sat, 16 Dec 2000 22:06:50 -0800

Me:
HELO localhost

Server:
250 localhost.localdomain Hello IDENT:mattrope@localhost [127.0.0.1],
pleased to meet you

Me:
MAIL From: somebody@somewhere.com

Server:
250 somebody@somewhere.com... Sender ok

(Note that the From: is bogus, but it doesn't matter, even on the
envelope.  From: doesn't affect how mail moves across the Internet)

Me:
RCPT To: mdroper@ucdavis.edu

(Note: this is the real email address and will be used on the envelope)

Server:
250 mdroper@ucdavis.edu... Recipient ok

Me:
DATA

Server:
354 Enter mail, end with "." on a line by itself

Now I get to type in the email, including the internal headers.~

Me:
To: myspamlist@places.com
From: johndoe@here.com
Subject: Spam

Test email
.

Server:
250 WAA02680 Message accepted for delivery

Me:
QUIT

Server:
221 localhost.localdomain closing connection


Now when the email gets to me, the email address myspamlist@places.com is
the apparent To: address.  mdroper@ucdavis.edu is nowhere to be found
because the envelope is discarded when the email reaches its final resting
spot.  Although the real To: is not visible, it turns out that the real
From: address is usually visible as the Return-Path: header in the
email.  So you can see both the internal and external From addresses, but
since they can both be spoofed, it doesn't do you any good.

That's my understanding of how this all works.  I'm no expert on mail
delivery, so something I said may be slightly off, but I think that's how
it works in general.


============================
Matt Roper
mdroper@ucdavis.edu
http://mattrope.tsx.org
============================



On Sat, 16 Dec 2000, Mark Kim wrote:

> Sometimes, like when I get a bulk e-mail, the e-mail address listed in the
> "To: " section is not my e-mail address.  Since the e-mail got to me,
> there must be some information in the header that tells to whom the e-mail
> was to be sent.  Does anybody know what this field is?  I can't find
> it...  Thanks!
> 
> -Mark
> 
> ---
> Mark K. Kim
> http://www.cbreak.org/mark/
> PGP key available upon request.
> 


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