[SpamCop-List] smtp dictionary attacks - always from "john@<randomdomain>" - who else is getting this?

[! Chris]

For the last few months, some persistent person/persons is
dictionary-attacking me - just 50 to 200 addresses - but every day, always
from "john@" something, always a new list of addresses, and almost always
from a COMPLETELY different IP address every time!

Is everyone else getting this?  Does anyone know anything more about it?  Is
this some kind of automated attack launched via "owned" (hacked) computers?
Is it some kind of mail scanning software that lots of different people are
running?

I noticed that they guessed one of my email addresses correctly a few days
ago; and a flood of colon-cleaner spam began to arrive...