A bug in nginx SMTP proxy was found, which allows an attacker in a privileged network position to inject commands into SSL sessions started with the STARTTLS command, potentially making it possible to steal sensitive information sent by clients (CVE-2014-3556).
The problem affects nginx 1.5.6 - 1.7.3.
The problem is fixed in nginx 1.7.4, 1.6.1.
Patch for the problem can be found here:
Thanks to Chris Boulton for discovering this.