I just looked in my bulk mail folder and found 2 of the blank emails with "no subject" in the subject line.

Here's the full header if that will be of any help:

 

Received: (qmail 1944 invoked by uid 30297); 17 Dec 2013 19:02:57 -0000
Received: from unknown (HELO IMP) ([68.178.213.15])
          (envelope-sender <newsletter@postoffice.renderosity.com>)
          by p3plsmtp11-03.prod.phx3.secureserver.net (qmail-1.03) with SMTP
          for <XXXX@XXXX>; 17 Dec 2013 19:02:57 -0000
Received: from ector.bondware.com ([66.18.125.139])
        by IMP with bizsmtp
        id 2j2w1n00530aPoJ01j2wWu; Tue, 17 Dec 2013 12:02:57 -0700
X-Authority-Analysis: v=2.0 cv=Vqf99ZKn c=1 sm=1
 a=PmSfyxl9Ad+Ur+q9bqVcsQ==:117
X-Spam: Blacklist<br></br><br></br>I've "X"d out my email address.<br></br><br></br>Thanks

"I reject your reality and substitute my own" - Adam Savage