I don't think I can look at their headers in full without opening them, which I'm worried about doing. I'm getting so many obviously from the same source, though, that I might get curious and open one soon a) to see what the hell it's all about and b) to see if there's any obvious address I can block.

None of them actually have attachments, which is what makes me assume they're spam rather than things carrying worms / viruses. Also, I have noticed they have started appearing with random words tacked on the end: e.g. 'why are you always offline terrain', which I presume is some kind of attempt to get past spam-filters.