It's also a common tactic for filtering inbound email.
Mention that people may optionally include some word like 'orange' in the subject line to tell you they've come via some place like your blog or whatever it may be, and have read at least carefully enough to notice this.
Of course ironically that trick's probably trivially broken now because of use of LLMs in spam. But the point stands, it's an old trick.
It's not so much a case of personal targeting or anything particularly deliberate.
LLMs are trained on the full internet. All relevant information gets compressed in the weights.
If your email and this instruction are linked on your site, that goes in there, and the LLM may with some probability decide it's appropriate to use it at inference time.
That's why 'tricks' like this may get broken to some degree by LLM spam, and trivially when they do, with no special effort on the spammer's part. It's all baked into the model.
What previously would have involved a degree of targeting that wouldn't scale now will not.
Apart from the fact that not even every human would read this and add it to the subject, this would still work.
I doubt there is any spam machine out there the quickly tries to find peoples personal blog before sending them viagra mail.
If you are being targeted personally, then of course all bets are off, but that would’ve been the case with or without the subject-line-trick