Questions about eSpam

[ThoughtCriminal]

Ok any experts here?

Like everyone, I get oodles of spam. Since I have websites where I once posted my email, probably more than most. Most contain the usual pitches for porn, meds, insurance, stock advice, etc. But many just baffle me. What is the purpose and origin for spams like this?:

- Subject header is "hi". Body has nothing, no text, no image, no attachments, just blank.

- Spam with no content except random lower case letters. For example: "nzzshpiihb"

My theory is that they are collecting email addresses from anyone who is foolish enought to reply or are just testing email lists for bounces.