Weird numbers emails???

I've received some questions on weird spam messages going around that look like this:

Subject: 586876

Body: 5556

or

Subject: 455

Body: 969

There seems to be lots of them going around, looking at some of the discussion on the topic at some newsgroups. The mails don't contain an attachment. There are only some strange numbers involved. The numbers keep changing though? So what are they? I'm not sure, but it might be that some botnet spammer is checking the quality of his email lists: finding out which messages bounce and which don't or maybe it was a glitch ... his finger pushed on the wrong button. Also spammers are humans aren't they?