Thousands of Mail delivery errors from different wordpress installations.

Has anyone have this problem?

As far as we can tell, it is just you that has a problem.

Here’s a few details.
I run my own vps.

WordPress 3.8

Here’s information about the Message error:

Return-path: <[email protected]>
Received: from rgcosme by server.cjteksolutions.com with local (Exim 4.82)
(envelope-from <[email protected]>)
id 1Vrfty-0007NG-4v
for [email protected]; Fri, 13 Dec 2013 22:28:26 -0500
To: [email protected]
Subject: WordPress Plugin
Date: Sat, 14 Dec 2013 03:28:26 +0000
From: WordPress <[email protected]>
Message-ID: <866f239df7350fc44671dff7d100f18b@##str_replacement[1]##>
X-Priority: 3
X-Mailer: PHPMailer 5.2.4 (https://code.google.com/a/apache-extras.org/p/phpmailer/)
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Content-Type: text/plain; charset=UTF-8

https://www.rgcosmetics.com

according to this Link

My guess is one of your installed plugins includes the base64 eval code that is mentioned at the bottom of the linked thread at stackexchange. Deactivate all plugins and see if error goes away.

Since you are running your own VPS, I would also strongly encourage you to review security setup on it and also review Hardening WordPress.

Find this code addressdecode=base64_decode("amFxcXNjaWdzQGdtYWlsLmNvbQ==") and then delete function. I had the same problem, this code was in template files.