[imp] Invalid Headers: unexpected end of preamble

[Fábio]

Hi there.
I'm facing some kind of problem with message headers encoding in IMP.
This error didn't happen 100% of the time, so after a long reseach by 
trial and error, this is what I've managed to figure out: It happens 
when I send a return receipt to a message that was sent both in HTML and 
plain text. Doesn't happen if the original msg is plain text only.
Any ideas?
T.I.A.

Here are the actual headers of the disposition-notification message 
generated by IMP 4.0.4:

Received: by (hostname deleted) (Postfix, from userid 99)
    id 5575A12E395; Wed, 21 Dec 2005 17:20:38 -0200 (BRST)
Message-ID: <20051221172037.eyj86hepsw4c0ww4@(hostname deleted)>
User-Agent: Horde Application Framework 3.0
Date: Wed, 21 Dec 2005 17:20:37 -0200
From: (deleted)
To: (deleted)
Subject: =?utf-8?b?Tm90aWZpY2HDp8Ojbw==?= de =?utf-8?b?RGlzcG9zacOnw6Nv?=
MIME-Version: 1.0
Content-Type: multipart/report;
    boundary="=_4ufpvyzu2tc0";
    report-type="disposition-notification"
Content-Transfer-Encoding: 7bit



Here is the error message from amavisd-new:

INVALID HEADER (INVALID CHARACTERS OR SPACE GAP)

MIME error: error: unexpected end of preamble

This nondelivery report was generated by the amavisd-new program
at host orion. Our internal reference code for your message
is 10393-04/WvvqQ+hC6elr.


WHAT IS AN INVALID CHARACTER IN MAIL HEADER?

  The RFC 2822 standard specifies rules for forming internet messages.
  It does not allow the use of characters with codes above 127 to be used
  directly (non-encoded) in mail header (it also prohibits NUL and bare CR).

  If characters (e.g. with diacritics) from ISO Latin or other alphabets
  need to be included in the header, these characters need to be properly
  encoded according to RFC 2047. This encoding is often done transparently
  by mail reader (MUA), but if automatic encoding is not available (e.g.
  by some older MUA) it is the user's responsibility to avoid the use
  of such characters in mail header, or to encode them manually. Typically
  the offending header fields in this category are 'Subject', 
'Organization',
  and comment fields in e-mail addresses of the 'From', 'To' and 'Cc'.

  Sometimes such invalid header fields are inserted automatically
  by some MUA, MTA, content checker, or other mail handling service.
  If this is the case, that service needs to be fixed or properly 
configured.
  Typically the offending header fields in this category are 'Date',
  'Received', 'X-Mailer', 'X-Priority', 'X-Scanned', etc.

  If you don't know how to fix or avoid the problem, please report it
  to __your__ postmaster or system manager.

Your message <20051221172037.eyj86hepsw4c0ww4@(hostname deleted) 
<javascript:open_compose_win('to=20051221172037.eyj86hepsw4c0ww4%40email.opus2.com.br&thismailbox=INBOX');>>
could not be delivered to:
   <(email address deleted) 
<javascript:open_compose_win('to=rferracin%40lactec.org.br&thismailbox=INBOX');>>:
   554 5.6.0 Message with invalid header rejected, id=10393-04 - MIME 
error: error: unexpected end of preamble