I have been getting a lot of bounce-backs lately.
I thought my google apps account has been compromised, but there is no activity on my apps account, and there is certainly no malicious user that I can see.
Since the email is always send from some random username (eg onSecNtV1@mydomain.com), I tried to find a way of disallowing sending email from unregistered users. I couldn't find anything.
What worries me is that the message has a X-Google-DKIM-Signature, and it says "Google tried to deliver your message". Does that mean the emails are originating from my trusted clients? (I only use gmail)
AFAIK spammers are free to spoof every field of the email, but the DKIM should make the (modern) servers drop these unverifiable emails.
Any idea where the leak is?
Here's an example bounce-back:
Delivery to the following recipient failed permanently: firstname.lastname@example.org Technical details of permanent failure: Google tried to deliver your message, but it was rejected by the server for the recipient domain 184.108.40.206 [220.127.116.11]. The error that the other server returned was: 553 sorry, that domain isn't in my list of allowed rcpthosts; no valid cert for gatewaying (#5.7.1) ----- Original message ----- X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20120113; h=x-received:x-received:received-spf:mime-version:date:message-id :from:to:subject:content-type:content-transfer-encoding :list-unsubscribe:x-gm-message-state; bh=l2RiLiEDvrHgBMSAjtLmNgIpmW4D1EFAIr3O42oBysM=; b=UBSW90YcP4Fu1vDLnvGCp06XEE5+FOAUR62qSnQrnaPcsKWJSdFT7x7XSU2+vHrpTI RaN4pHJWlaMHqtAUoMFE0T9hgBj0blZnNMDtMRFkcU4QD0E/QNw6VIQlAjWGOWvXghMc G+SX+YLugnQEWS6tG6guf1hF31XoB4a2HxvxQO4J+lWNLg60LaS7K4DiUr4yG25mvXBU uy+tXqjLKyZgA9jmvyVvBKeRYVwMIWvscJ26yw17K7LRfGZkAXzuvTVyGMuLUzthj5c5 MSNZOG6u5faxtzdBkGRiNQVarq3IsXBuXcxk1vRiUktbM8OIhm2D4IrvhmTPrDF4yyTz EHhw== X-Received: by 10.50.37.239 with SMTP id b15mr4892361igk.69.1360854627245; Thu, 14 Feb 2013 07:10:27 -0800 (PST) X-Received: by 10.50.37.239 with SMTP id b15mr4892357igk.69.1360854627177; Thu, 14 Feb 2013 07:10:27 -0800 (PST) Return-Path: <onSecNtV1@mydomain.com> Received: from [18.104.22.168] ([22.214.171.124]) by mx.google.com with ESMTP id vx6si33676538igb.26.2013.02.14.07.10.25; Thu, 14 Feb 2013 07:10:26 -0800 (PST) Received-SPF: neutral (google.com: 126.96.36.199 is neither permitted nor denied by best guess record for domain of onSecNtV1@mydomain.com) client-ip=188.8.131.52; Authentication-Results: mx.google.com; spf=neutral (google.com: 184.108.40.206 is neither permitted nor denied by best guess record for domain of onSecNtV1@mydomain.com) smtp.mail=onSecNtV1@mydomain.com MIME-Version: 1.0 Date: Thu, 14 Feb 2013 20:40:29 +0530 Message-ID: <B23FC935D84FBB6D39DD9BDCC44CD2.176.3699432888759@D> From: "Valetine's Day" <onSecNtV1@mydomain.com> To: email@example.com Subject: I will be excited if you are my Valetine Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: 8bit List-Unsubscribe: <mailto:E7E740C2663A5B2B9D18@missinyou.com> X-Gm-Message-State: ALoCoQkVcAw4pG/8g2x1C02KHf4lLkBdJ4iLe9r1ZeGlGE1AwtZEQm5VsHh9tNmG04yH2ahWqXnIiKu7DrTf7j6bLHEaF0l0AMhrC6ZvnyJTUr4n+9TKMieQPycP0Pw8sCJ8DELiMNlLI/CGbgQ1ObMLghXauZTeqg== ----- End of message -----