What is with this SPAM?

For the last few days, I have been receiving these ridiculous SPAM emails. They all are different, although they all follow the same pattern. They all have a short nonsense subject (like “rb 4d”) and then a short nonsense body (like “wn”). That’s it. I get them 3 and 4 at a time. What is the point?

I always thought they were ‘feelers’ for legit email addresses. Checking for bounce backs/undeliverables and people who actually respond (marks).