Hound, Etherfast, thanks for your replies. Sorry I posted this in the wrong category.
I found the source of the spam (a unix mailing list user, who keeps sending 5 or 6 emails per day. And it seems my email address still resides in posts on the mailing list, even though I'd unsubscribed ages ago. Some spammer(s) figured out that they could use firstname.lastname@example.org
The spam started arriving just after I'd registered an Undernet username, which made me wonder.
Anyway, I'm glad it's not from Undernet, as it's a good medium.
Thanks for your help!