Frank Ellermann writes:
... this spammer failed to inform me why he started to abuse
xyzzy.claranet.de resulting in 1000 backscatter mails per day for
about six moths, and s/h/it also failed to inform me why s/h/it
Two other plausible reasons except from "got the SPF idea":
I've been the victim of major backscatter several times. Five or ten,
perhaps more, I don't know any more. Once I got 30,000 bounces in just
a few days. It usually went on for a while, then stopped suddenly.
I suspect that every few weeks, they just pick a new victim. Nothing to
do with the victim's actions or inaction.