Use SMTP not Sendmail

Here the headers of a mail sent when using Horde_Mail 1.1.0, the 
result is the same when using 1.0.3. Naturally the mail is unreadable 
by any mail clients with this format, the body appearing somewhere in 
the headers.

Downgrading to Horde_Mail 1.0.2 fixes the problem and mail can be sent 
in the correct way again. Any debugging strategies I could use to find 
out what's wrong with this would be gratefully used.

Domain names have been changed to protect the innocent:

Return-path: <ben@example.net>
Envelope-to: bengreen@localhost
Delivery-date: Sat, 17 Mar 2012 11:08:55 +0000
Received: from localhost ([127.0.0.1])
      by received.co.uk with esmtp (Exim 4.72)
      (envelope-from <ben@example.net>)
      id 1S8rVH-0000Ds-Pr
      for bengreen@localhost; Sat, 17 Mar 2012 11:08:55 +0000
X-Quarantine-ID: <fhs4Dp2rTM5r>
X-Virus-Scanned: Debian amavisd-new at otherexample.net
X-Amavis-Alert: BAD HEADER SECTION, Improper folded header field made up
      entirely of whitespace (char 20 hex): Received: ...tent-Disposition:
      inline\n \n yes, a fart\n [...]
X-Spam-Flag: NO
X-Spam-Score: -4.914
X-Spam-Level:
X-Spam-Status: No, score=-4.914 tagged_above=-999 required=4
      tests=[AM:BOOST=-7, AWL=-1.155, BAYES_00=-1.9, EMPTY_MESSAGE=2.32,
      MISSING_HEADERS=1.021, MISSING_SUBJECT=1.799,
      MSGID_FROM_MTA_HEADER=0.001] autolearn=no
Received: from received.co.uk ([127.0.0.1])
      by localhost (received.co.uk [127.0.0.1]) (amavisd-new, port 10024)
      with ESMTP id fhs4Dp2rTM5r for <bengreen@localhost>;
      Sat, 17 Mar 2012 11:08:55 +0000 (GMT)
Received: from mailserv.psand.net ([194.164.97.5])
      by received.co.uk with esmtps (TLS1.0:RSA_AES_256_CBC_SHA1:32)
      (Exim 4.72)
      (envelope-from <ben@example.net>)
      id 1S8rVG-0000Dn-Vv
      for bengreen@otherexample.net; Sat, 17 Mar 2012 11:08:54 +0000
Received: from www-data by mailserv.psand.net with local (Exim 4.72)
      (envelope-from <ben@example.net>)
      id 1S8rVG-0003c6-0x
      for bengreen@otherexample.net; Sat, 17 Mar 2012 11:08:54 +0000
Date: Sat, 17 Mar 2012 11:08:54 +0000
Message-Id: <E1S8rVG-0003c6-0x@mailserv.psand.net>
Received: from cpc10-aztw24-2-0-cust114.aztw.cable.virginmedia.com
      (cpc10-aztw24-2-0-cust114.aztw.cable.virginmedia.com [82.45.35.115]) by
      slackmail.co.uk (Horde Framework) with HTTP; Sat, 17 Mar 2012 
11:08:53 +0000
      Date: Sat, 17 Mar 2012 11:08:53 +0000
      Message-ID: 
<20120317110853.Horde.bhzyWWOelG9PZHDF8uVRJ7A@slackmail.co.uk>
      From: Ben Green <ben@example.net>
      To: "bengreen@otherexample.net" <bengreen@otherexample.net>
      Subject: this is the subject of the message
      User-Agent: Internet Messaging Program (IMP) H4 (5.0.19)
      Content-Type: text/plain; charset=ISO-8859-1; format=flowed; DelSp=Yes
      MIME-Version: 1.0
      Content-Disposition: inline
      This is the body of the message, and my signature.
      ==
      From Ben Green
From: www-data <ben@example.net>