This APPEARS TO BE SPAM and the best thing to do is to delete them immediately!
There is a suggestion that these come from a spammer trying to validate an email address. See:
http://www.ehmac.ca/anything-mac/3291-i-keep-getting-these-blank-emails-what.html
From the Apple Discussion Boards:
http://discussions.apple.com/thread.jspa?threadID=1925198&tstart=0
There is a suggestion to set up a rule to move them to junk (personally I don't like this, I shouldn't have to set up rules to deal with spam/system errors). This Apple thread also seems to confirm that it's MobileMe related.
Not really much of an answer...