Do you use Outlook 2003? If yes, that's might be the problem. Outlook tend to freeze, and stop downloading new messages when a message with faulty header comes in. The blank mails you get might be the symptom for that problem .

The other option is that spammers are looking for valid emails by implanting a tiny image which can't be seen. By opening the message you actually hitting the image, and that's an indication for spammers that your email is alive and valid.