How to prevent spam from "as permitted sender" senders?

Question

Looking into my gmail spam folder, I see many messages coming from an old domain.

How does it work, that I receive spam from an old account? And, how can I stop these spammers of using the old email address?

The original message looks like:

Delivered-To: [email protected]
Received: by 10.112.2.99 with SMTP id 3csp17644lbt;
        Sat, 25 Oct 2014 03:21:10 -0700 (PDT)
X-Received: by 10.50.20.130 with SMTP id n2mr9275776ige.44.1414232469601;
        Sat, 25 Oct 2014 03:21:09 -0700 (PDT)
Return-Path: 
Received: from m68-68.mailgun.net (m68-68.mailgun.net. [166.78.68.68])
        by mx.google.com with ESMTPS id r14si9570045ici.56.2014.10.25.03.21.08
        for 
        (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128);
        Sat, 25 Oct 2014 03:21:09 -0700 (PDT)
Received-SPF: pass (google.com: domain of [email protected] designates 166.78.68.68 as permitted sender) client-ip=166.78.68.68;
Authentication-Results: mx.google.com;
       spf=pass (google.com: domain of [email protected] designates 166.78.68.68 as permitted sender) [email protected];
       dkim=pass [email protected]
DKIM-Signature: a=rsa-sha256; v=1; c=relaxed/relaxed; d=olddomain.de; q=dns/txt;
 s=smtp; t=1414232468; h=Sender: Content-Transfer-Encoding:
 Content-Type: Mime-Version: Date: Subject: To: From: Message-Id:
 X-Feedback-Id; bh=9g2dkbMafTPPEBO0f5+XleX+GeXgX2IeYFkI8dNbUOw=; b=CYkMNNIx52f2mBhnhMmGabiRZc9WoQarUUpStb/oX1O9CkasFaOjATliSHiinnym7jeSE0R7
 PpzI2lWjpN0tsqPAkfD4O8mUSajXeDkBX3wirDN+XD1IB7da/f+7kIu5NWs1yA4/roY0Vp1c
 Oqi1gbRUDjWZ6BiViwo8TmoN6+Y=
DomainKey-Signature: a=rsa-sha1; c=nofws; d=olddomain.de; s=smtp;
 q=dns; h=X-Feedback-Id: Message-Id: From: To: Subject: Date:
 Mime-Version: Content-Type: Content-Transfer-Encoding: Sender;
 b=dDCbln699Wbl/WmYC+547/stjF/nMbgzZ6nJq8y4EwF/HemVY+pERNWowb+jONIASno9HV
 hycDs0SFxz2mbJVYBb2vSqWQOVUSsItgJy1shFApLKiDQI1sU5kh4tf0DjEQO8fpYJbtk2VI
 PMKx/VEJ9XDDcJJqLpXB026th5FJA=
DKIM-Signature: a=rsa-sha256; v=1; c=relaxed/relaxed; d=mailgun.org; q=dns/txt; s=mg;
 t=1414232468; h=Sender: Content-Transfer-Encoding: Content-Type:
 Mime-Version: Date: Subject: To: From: Message-Id: X-Feedback-Id;
 bh=9g2dkbMafTPPEBO0f5+XleX+GeXgX2IeYFkI8dNbUOw=; b=J2HHyT6pnuo9kdTCqyryLqPKzeLIs3OGk72lh1qwLmOUUAUE5HlG//pFdxKQImerS8A1z3bk
 BCdf6fq15n61ptsAsKR3FOPI7CaCHzzMXhJnMqDs93VApjv7DyvZCGF9H/qyyRQ06LXXIDi2
 l6B6JgHSOnkIe6z9abIe4UtLrtg=
DomainKey-Signature: a=rsa-sha1; c=nofws; d=mailgun.org; s=mg; q=dns;
 h=X-Feedback-Id: Message-Id: From: To: Subject: Date: Mime-Version:
 Content-Type: Content-Transfer-Encoding: Sender;
 b=e2n7aAAJVPM5GC1Blz5qT+qccePpRsYqM1Ed3Gt5Dm/UuWmC6bediTSrBcpRiujP0BUbt3
 H/9NCZw5HTK/j8b3xYlnQqsDvBgjoC29iDFGe93hD2Td7AmUJcbRbGy3U9zXqG+aCqJBq8fm
 t5bF/+2yoY676KM1nQAnYMcL+gTD0=
X-Feedback-Id: 52039716dcf54642f13fdbad:mailgun
X-Envelope-From: 
Received: from [77.30.245.180] (Unknown [77.30.245.180]) by mxa.mailgun.org
 with ESMTP id 544b798d.7fde005e64c8-in2; Sat, 25 Oct 2014 10:21:01 -0000
 (UTC)
Message-Id: 
From: "[email protected]" 
To: 
Subject: Chheap Pharmaacy
Date: 25 Oct 2014 14:43:46 +0200
Mime-Version: 1.0
Content-Type: text/plain; charset="iso-8859-1"
Content-Transfer-Encoding: 7bit
X-Mailer: Byluam apckfs 9.0
X-Mailgun-Incoming: Yes
X-Mailgun-Sid: WyI5YWVmNiIsICJtdWxkZXIucGF0cmlja0BnbWFpbC5jb20iLCAiMTMyMjYiXQ==
Sender: [email protected]

Medsstore che@p
http://purecanadianshop.ru/?dvowignru|

Answer 1

The "From" header in the email header can be forged. You can't change that, but the DKIM header allows a machine to sign the content and some headers, and that signature can then be validated against a public key published in dns. Google then adds the "Authentication-Results" header which describes the results of checking that signature.

It looks like this email has been legitimately signed.

It's hard for me to check the formatting of the email carefully (on my phone), but it looks like this might be a bounce message? E.g. mail sent to your old address, which bounces back to your new one. I guess your new address was the apparent sender of the mail that bounced.