Return-path double, set incorrectly in mass mailings

When having Outbound Mail Settings set to "Sendmail" the emails received in mass emailings have two return-paths, with one of them being equal to the bounce email box and the other equal to the from address. When the setting is changed to SMTP, then again there are two return-paths in the message, but both are the bounce return path.

My mail server uses Qmail and is on a different server than civicrm. The sendmail on the civicrm server is sSMTP. It is, of course, quite possible that the issue is in sSMTP instead of civicrm.

It seems odd that there are two return paths being made.

Headers when set to "Sendmail" (I've replaced .net and .sk in email addresses here to prevent spam harvesting...)

Return-Path: <info@cbreurope.***>
Delivered-To: andrew@freedomlives.***
Received: (qmail 11685 invoked from network); 3 Jul 2012 03:43:56 -0000
Received: from unknown (HELO sk) (bounces@37.205.11.55)
by mail.freedomlives.net with ESMTPA; 3 Jul 2012 03:43:56 -0000
Received: by sk (sSMTP sendmail emulation); Tue, 03 Jul 2012 05:35:47 +0200
MIME-Version: 1.0
Content-Type: multipart/alternative;
boundary="=_d0886896795e4fe9d3c8698bbbe7a80c"
Reply-To: "CBR" <info@cbreurope.**>
Return-Path: bounces-b.165.170927.9ed330d8d87950c8@cbreurope.sk
From: "CBR" <info@cbreurope.**>
Subject: =?utf-8?Q?prolife_dobrovo=C4=BEn=C3=ADci_v_Nitre=2C_Tren=C4=8D?=
=?utf-8?Q?=C3=ADne=2C_Levo=C4=8Di?=
List-Unsubscribe: <bounces-u.165.170927.9ed330d8d87950c8@cbreurope.sk>
To: <andrew@freedomlives.***>
Precedence: bulk
X-CiviMail-Bounce: bounces-b.165.170927.9ed330d8d87950c8@cbreurope.**
Date: Tue, 03 Jul 2012 05:35:47 +0200

Headers when sent with SMTP:
Return-Path: <bounces-b.167.174329.f600cec174db84e0@cbreurope.sk>
Delivered-To: andrew-info@freedomlives.***
Received: (qmail 627 invoked from network); 4 Jul 2012 08:58:21 -0000
Received: from unknown (HELO localhost) (bounces@37.205.11.55)
by mail.freedomlives.net with ESMTPA; 4 Jul 2012 08:58:21 -0000
MIME-Version: 1.0
Content-Type: multipart/alternative;
boundary="=_4573f6f3c3fbdb43007cb11561f28a90"
Reply-To: "CBR" <info@cbreurope.**k>
Return-Path: bounces-b.167.174329.f600cec174db84e0@cbreurope.sk
From: "CBR" <info@cbreurope.**>
Subject: Test Email
List-Unsubscribe: <bounces-u.167.174329.f600cec174db84e0@cbreurope.sk>
To: Andrew Ray <andrew-info@freedomlives.***>
Precedence: bulk
X-CiviMail-Bounce: bounces-b.167.174329.f600cec174db84e0@cbreurope.sk
Date: Wed, 04 Jul 2012 10:50:03 +0200