What is the mail client you use to send the mail?
Bug 3146 is about EAS smartForward/smartReply and has nothing to do with what you describe above.

We are using the web-based interface, and email forwarding is handled via Dovecot Managesieve.

How looks the mail in senders sent-folder?
The raw-source of the mail and screenshots can help to better understand the problem.

Original Message Source (sanitized):

Return-Path: <user@gmail.com>
Received: from mail2.domain.com
by mail2.domain.com (Dovecot) with LMTP id FA71B+cv31VaGAAA3OIF/Q
; Thu, 27 Aug 2015 21:13:17 +0530
Received: from mx2.domain.net (mx2.domain.net [74.xxx.xxx.xxx])
by mail2.domain.com (Postfix) with ESMTPS id D5A8FD180557
for <user@domain.com>; Thu, 27 Aug 2015 21:13:16 +0530 (IST)
Received: from mail-pa0-f45.google.com ([209.85.220.45])
by mx2.domain.net with esmtps (TLSv1.2:RC4-SHA:128)
(Exim 4.85)
(envelope-from <user@gmail.com>)
id 1ZUzJ8-0003nv-9q
for user@domain.com; Thu, 27 Aug 2015 11:41:48 -0400
Received: by pacgr6 with SMTP id gr6so11678445pac.3
for <user@domain.com>; Thu, 27 Aug 2015 08:41:41 -0700 (PDT)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
d=gmail.com; s=20120113;
h=from:to:subject:date:message-id:mime-version:content-type
:content-language:thread-index;
bh=sSySlqHx9Vf/+MEV/Zo67nbPykHilOGwOlTJ45NwpOI=;
b=rYuts5ibRTChNFSG7u1h2UyngP+/puL41m2fyCdtKNk59FkcrWTroZv7OTm0izAkf6
nvk8rJWz997usAE3SY8Jaln5L+3YbA0gW9wDoQw+SgdKZjVGgrHfe8uexIMC4jw/GFxa
EXGF74W4gjCGDCRdlydI1WqtW0h/UohSyUDy8d8XZgIMcf2PT+AoGBqVJ89dXF3GbScw
fnwopZw44EOdvj2fuucxbf2tw/2FucCUOaw+ATrqW+Dfeh/ebMWZTat9alP0sajaZwO4
qEBT27Gil6zEj7jNFhUAY6rs0BYGuOTrlUid5hvrUMMT+G7UgDKeq7eE1Z/Jcf3iFgEe
IWAg==
X-Received: by 10.69.12.33 with SMTP id en1mr7500821pbd.97.1440690100949;
Thu, 27 Aug 2015 08:41:40 -0700 (PDT)
Received: from Dell ([124.xxx.xxx.xxx])
by smtp.gmail.com with ESMTPSA id w6sm2832832pdn.46.2015.08.27.08.41.38
for <user@domain.com>
(version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128);
Thu, 27 Aug 2015 08:41:40 -0700 (PDT)
From: "User" <user@gmail.com>
To: <user@domain.com>
Subject: Test
Date: Thu, 27 Aug 2015 23:41:39 +0800
Message-ID: <871d01d0e0de$dfb70720$9f251560$@gmail.com>
MIME-Version: 1.0
Content-Type: multipart/alternative;
boundary="----=_NextPart_000_871E_01D0E121.EDDAE360"
X-Mailer: Microsoft Outlook 15.0
Content-Language: en-us
Thread-Index: AdDg3tfggDDX/PkOS+WcITuEDRBEsQ==
X-Antivirus: avast! (VPS 150827-0, 27/08/2015), Outbound message
X-Antivirus-Status: Clean
Received-SPF: pass (mx2.domain.net: domain of gmail.com designates 209.85.220.45 as permitted sender) client-ip=209.85.220.45; envelope-from=user@gmail.com; helo=mail-pa0-f45.google.com;
X-SPF-Result: mx2.domain.net: domain of gmail.com designates 209.85.220.45 as permitted sender
Authentication-Results: mx2.domain.net; dmarc=pass header.from=gmail.com
X-Filter-ID: s0sct1PQhAABKnZB5plbIcmfyoaeJJA5U1wYGh1SeFbgqdjY83C8F3nyJIBATic1ZlPPEMQ1sApk
7kcHVTPcfQ4nmWJKt/vt1/Gk+iJgp2ES1p4Y1RXkElkx0pKW4Xs0ERWeKKG4PAQYNyavp7c49JS5
RagvVWAtx/zKdQ/6dt+ncmiK7Zco8MPBEpKMNy1clS3pcIM8/bcvSE3kQDC5Mqvgy/D14lSjIuYH
FMoDHQQ8AJMaBatHnafPiKcKL4/BYiAGRFbuLCtSljQo8fbqDrBzuwIHCgNdbAV1uUpVVsRgVotb
oCnB36SmCfkj8hz85YE5enyccp7RH4WQio3uGUphUOTCuwQEGihbqDh0KJ51gMljEk6Qcq6tXqK2
7d+QcuhY9nYDlc78I3zsDk/+3Do6vZ0VzWcY+Mv3BrFZ0OoAU59dWkSYIxACD1p54eJk3N3mR7B+
tJwrUc6LIc0nTudnxqIWJb2VLgXRKBQlc7CWcRpN1+1Pwq6Xpmk2BKo08bczhdhMiuGc7RKtcaWB
EsMjh2a9N+rSiVn95dKlohwIo/0CJiIFjM9L39GSltgFrQkqqQ1qQTTN7wbrTBAnbfY4ocfmWv3F
e9Iziczdq+A=
X-Report-Abuse-To: spam@mx1.domain.net
X-Filter-Fingerprint: IFrWXGses7OKB5S5G8/dJSzJ+nIQaeys2NdRY49AYNRA3cTUQ1R++keuE7RDJ8Kg3RbMLUalw1oC
mj99/u+Pol7jVop6A9jB5e0iklqxH6Kok1JBYnOLzfRIhlEHQynLUpndEJ0YoaLytXXo8BMTaX2p
Mk7LBarWD9Fj4R3eIu6hRW81gsiK49JiuSr1ZvUvLXDeuFcjvKlYdwLNF4uyRJ6jB2zM4iaFfVNC
PlXIN7PkWYsLPMmWrsjCizhiEGsW
Authentication-Results: domain.net; spf=pass smtp.mailfrom=user@gmail.com
Authentication-Results: domain.net; dkim=pass header.i=gmail.com
X-SpamExperts-Class: ham
X-SpamExperts-Evidence: Combined (0.15)
X-Recommended-Action: accept

This is a multipart message in MIME format.

------=_NextPart_000_871E_01D0E121.EDDAE360
Content-Type: text/plain;
charset="US-ASCII"
Content-Transfer-Encoding: 7bit

Test

This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

------=_NextPart_000_871E_01D0E121.EDDAE360
Content-Type: text/html;
charset="US-ASCII"
Content-Transfer-Encoding: quoted-printable

<html xmlns:v=3D"urn:schemas-microsoft-com:vml" xmlns:o=3D"urn:schemas-micr=
osoft-com:office:office" xmlns:w=3D"urn:schemas-microsoft-com:office:word" =
xmlns:m=3D"http://schemas.microsoft.com/office/2004/12/omml&quot; xmlns=3D"http:=
//www.w3.org/TR/REC-html40"><head><META HTTP-EQUIV=3D"Content-Type" CONTENT=
=3D"text/html; charset=3Dus-ascii"><meta name=3DGenerator content=3D"Micros=
oft Word 15 (filtered medium)"><style><!--
/ Font Definitions /
@font-face
{font-family:"Cambria Math";
panose-1:2 4 5 3 5 4 6 3 2 4;}
@font-face
{font-family:Calibri;
panose-1:2 15 5 2 2 2 4 3 2 4;}
/ Style Definitions /
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0in;
margin-bottom:.0001pt;
font-size:11.0pt;
font-family:"Calibri",sans-serif;
mso-fareast-language:EN-US;}
a:link, span.MsoHyperlink
{mso-style-priority:99;
color:#0563C1;
text-decoration:underline;}
a:visited, span.MsoHyperlinkFollowed
{mso-style-priority:99;
color:#954F72;
text-decoration:underline;}
span.EmailStyle17
{mso-style-type:personal-compose;
font-family:"Calibri",sans-serif;
color:windowtext;}
=2EMsoChpDefault
{mso-style-type:export-only;
font-family:"Calibri",sans-serif;
mso-fareast-language:EN-US;}
@page WordSection1
{size:8.5in 11.0in;
margin:1.0in 1.0in 1.0in 1.0in;}
div.WordSection1
{page:WordSection1;}
--></style><!--[if gte mso 9]><xml>
<o:shapedefaults v:ext=3D"edit" spidmax=3D"1026" />
</xml><![endif]--><!--[if gte mso 9]><xml>
<o:shapelayout v:ext=3D"edit">
<o:idmap v:ext=3D"edit" data=3D"1" />
</o:shapelayout></xml><![endif]--></head><body lang=3DEN-SG link=3D"#0563C1=
" vlink=3D"#954F72"><div class=3DWordSection1><p class=3DMsoNormal><b><span=
style=3D'font-size:14.0pt;color:red'>Test<o:p></o:p></span></b></p></div>
<br /><br />
<hr style=3D'border:none; color:#909090; background-color:#B0B0B0; height: =
1px; width: 99%;' />
<table style=3D'border-collapse:collapse;border:none;'>
<tr>
<td style=3D'border:none;padding:0px 15px 0px 8px'>
<a href=3D"https://www.avast.com/antivirus&quot;>
<img border=3D0 src=3D"http://static.avast.com/emails/avast-mail-stamp.=
png" alt=3D"Avast logo" />
</a>
</td>
<td>
<p style=3D'color:#3d4d5a; font-family:"Calibri","Verdana","Arial","Helv=
etica"; font-size:12pt;'>
This email has been checked for viruses by Avast antivirus software.
<br><a href=3D"https://www.avast.com/antivirus&quot;>www.avast.com&lt;/a>
</p>
</td>
</tr>
</table>
<br />
</body></html>
------=_NextPart_000_871E_01D0E121.EDDAE360--

Forwarded Email (Sanitized):

Return-path: <user@gmail.com>
Envelope-to: user@domain.net
Delivery-date: Thu, 27 Aug 2015 10:46:57 -0500
Received: from [74.119.146.184] (port=48949 helo=mx2.domain.net)
by megami.domain.net with esmtps (TLSv1.2:DHE-RSA-AES256-GCM-SHA384:256)
(Exim 4.85)
(envelope-from <user@gmail.com>)
id 1ZUzOC-0005EB-PD
for user@domain.net; Thu, 27 Aug 2015 10:46:56 -0500
X-DKIM-Failure: bodyhash_mismatch
Received: from mail2.domain.com ([199.xxx.xxx.xxx])
by mx2.domain.net with esmtps (TLSv1.2:DHE-RSA-AES256-SHA:256)
(Exim 4.85)
(envelope-from <user@gmail.com>)
id 1ZUzOA-0004Bd-6V
for user@domain.net; Thu, 27 Aug 2015 11:46:57 -0400
Received: by mail2.domain.com (Postfix, from userid 1302)
id 90359D1806E9; Thu, 27 Aug 2015 21:18:21 +0530 (IST)
X-Sieve: Pigeonhole Sieve 0.4.2
X-Sieve-Redirected-From: user@domain.com
Received: from mail2.domain.com
by mail2.domain.com (Dovecot) with LMTP id tW5hDUUx31V9GgAA3OIF/Q
; Thu, 27 Aug 2015 21:18:21 +0530
Received: from mx2.domain.net (mx2.domain.net [74.xxx.xxx.xxx])
by mail2.domain.com (Postfix) with ESMTPS id B6E64D1807BC
for <user@domain.com>; Thu, 27 Aug 2015 21:18:20 +0530 (IST)
Received: from mail-pa0-f44.google.com ([209.85.220.44])
by mx2.domain.net with esmtps (TLSv1.2:RC4-SHA:128)
(Exim 4.85)
(envelope-from <user@gmail.com>)
id 1ZUzAQ-0002xN-As
for user@domain.com; Thu, 27 Aug 2015 11:32:45 -0400
Received: by pacdd16 with SMTP id dd16so29150191pac.2
for <user@domain.com>; Thu, 27 Aug 2015 08:32:40 -0700 (PDT)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
d=gmail.com; s=20120113;
h=from:to:subject:date:message-id:mime-version:content-type
:content-language:thread-index;
bh=eQgZqPWNsOhRQhdivlUhuMT550ey/H3ItJM++bDP2LA=;
b=HbMrLpGZdLyL/tkF7YdNJKU2h65slG8olZiuoP6QIhMn3KhRpdRs76rZZESPsbAxgU
Hyb8tq62fhXPnoaj3PD0sSaZx/dYwPKFPLtcvIMQF4Qk4JimryL0htOrmJU18HXDAbhN
pCuKPcFpH4RyXZVuJQxblllzjMuxFQewLYBzbP3DEfMJaLZuWqPcLLr3bjiXEYzjrH2P
UOSIOfJmaCs4Y56ow/Zl8L1n0UEWqeP0wF2BWUD1O3hDd5WoAgXMoRAoUbrZzeSjpDTm
opmtYEJit9RtM6snj9n+EB+hIOZJh1wpNAqqk6UzrAlx42mps5buWkgsF7mraHjFACKh
/uwQ==
X-Received: by 10.68.98.5 with SMTP id ee5mr7419717pbb.95.1440689560712;
Thu, 27 Aug 2015 08:32:40 -0700 (PDT)
Received: from Dell ([124.xxx.xxx.xxx])
by smtp.gmail.com with ESMTPSA id rd8sm2823429pdb.10.2015.08.27.08.32.38
for <user@domain.com>
(version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128);
Thu, 27 Aug 2015 08:32:40 -0700 (PDT)
From: "USER" <user@gmail.com>
To: <user@domain.com>
Subject: Test
Date: Thu, 27 Aug 2015 23:32:38 +0800
Message-ID: <86dc01d0e0dd$9dd1f000$d975d000$@gmail.com>
MIME-Version: 1.0
Content-Type: multipart/alternative;
boundary="----=_NextPart_000_86DD_01D0E120.ABF5CC40"
X-Mailer: Microsoft Outlook 15.0
Content-Language: en-us
Thread-Index: AdDg3Zk0+bePG3dXT9u5nTMHg6aGnw==
X-Antivirus: avast! (VPS 150827-0, 27/08/2015), Outbound message
X-Antivirus-Status: Clean
Received-SPF: pass (mx2.domain.net: domain of gmail.com designates 209.85.220.44 as permitted sender) client-ip=209.85.220.44; envelope-from=user@gmail.com; helo=mail-pa0-f44.google.com;
X-SPF-Result: mx2.domain.net: domain of gmail.com designates 209.85.220.44 as permitted sender
Authentication-Results: mx2.domain.net; dmarc=pass header.from=gmail.com
X-Filter-ID: s0sct1PQhAABKnZB5plbIcmfyoaeJJA5U1wYGh1SeFbgqdjY83C8F3nyJIBATic1ZlPPEMQ1sApk
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e9Iziczdq+A=
X-Report-Abuse-To: spam@mx1.domain.net
X-Filter-Fingerprint: IFrWXGses7OKB5S5G8/dJSzJ+nIQaeys2NdRY49AYNRA3cTUQ1R++keuE7RDJ8Kg3RbMLUalw1oC
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Authentication-Results: domain.net; spf=pass smtp.mailfrom=user@gmail.com
Authentication-Results: domain.net; dkim=pass header.i=gmail.com
X-SpamExperts-Class: ham
X-SpamExperts-Evidence: Combined (0.15)
X-Recommended-Action: accept
Received-SPF: softfail (mx2.domain.net: transitioning domain of gmail.com does not designate 199.187.211.55 as permitted sender) client-ip=199.187.211.55; envelope-from=user@gmail.com; helo=mail2.domain.com;
X-SPF-Result: mx2.domain.net: transitioning domain of gmail.com does not designate 199.187.211.55 as permitted sender
Authentication-Results: mx2.domain.net; dmarc=none header.from=gmail.com
X-Filter-ID: s0sct1PQhAABKnZB5plbIcmfyoaeJJA5U1wYGh1SeFbgqdjY83C8F3nyJIBATic10JuF70A4EJMc
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X-Report-Abuse-To: spam@mx1.domain.net
X-Filter-Fingerprint: IFrWXGses7OKB5S5G8/dJdIz5bb8V0ykx8BnFBnunHBA3cTUQ1R++keuE7RDJ8Kg3RbMLUalw1oC
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Authentication-Results: domain.net; spf=softfail smtp.mailfrom=user@gmail.com
Authentication-Results: domain.net; dkim=fail (bodyhash_mismatch) header.i=gmail.com
X-SpamExperts-Class: ham
X-SpamExperts-Evidence: Combined (0.15)
X-Recommended-Action: accept
X-Antivirus: avast! (VPS 150827-0, 27/08/2015), Inbound message
X-Antivirus-Status: Clean

Content-Type: text/plain;

charset="US-ASCII"

Content-Transfer-Encoding: 7bit

Test

This email has been checked for viruses by Avast antivirus software.

https://www.avast.com/antivirus

If I understand correctly, you send mail from gmail to your server (postfix?) which delivers to dovecot which redirects (sieve) the mail to its destination.

If this is correct sogo is not involved here after creating the sieve-rule and it seems to be an issues between postfix, dovecot and sieve.

See following thread for a similar issue:
http://www.dovecot.org/list/dovecot/2014-January/094461.html

Hi. I am testing using Gmail as an example. This happens for all of the forwarded HTML emails. We are using the Managesieve built into Dovecot which processes the sieve script saved by SOGo in the user's home directories and forwards the emails as required.

I did saw that thread earlier today, and was testing upgrading Postfix to see if that will solve the issue, but it broke other components of the setup, so had to revert back for now.

Will test this further and report back.

Does not seem to be involving SOGo.