When is 'for <recipient>'present in header?

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When is 'for <recipient>'present in header?
\ Francisco Reyes (16 Apr 2007)
. \ Ralf Hildebrandt (16 Apr 2007)
. \ Victor Duchovni (16 Apr 2007)

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Subject:	When is 'for <recipient>'present in header?
Group:	Postfix-users
From:	Francisco Reyes
Date:	16 Apr 2007

Is there always supposed to be a "for <recipient>" component in a message
header?

Have a user that has set some filtering based on "for <recipient>" and he
noticed recently that many emails did not have it in the particular server
where we host his domain.

It is an old server running postfix 2.2.3 and we plan to move all customers
out of it so if it is an issue with that version of Postfix we will just
offer to move the domain to a different server. However, looking at several
other servers with newer versions of postfix, 2.2.10 and 2.3.8, I also see
many messages that do not have that in the header.

I tried to look for a pattern, but couldn't find one.. as to when "for
<recipient>" would appear on the header and when not.

To clarify, sample subset of header with "for <recipient>" follows.

Received: from mail13.domain.com (localhost [127.0.0.1])
by mail13.domain.com (Postfix) with ESMTP id 47ED877B371
for <Jonny>; Fri, 16 Mar 2007 10:31:05 -0400 (EDT)