[thombat]

But how did recipient A get a personalised copy if the mail with that copy was sent to a list of recipients?

Edit: WP has the Tesla story with the counsel forwarding his copy to everyone in a new mail (presumably trying to be helpful?) So not a case of reply-all disease

https://en.m.wikipedia.org/wiki/Canary_trap

[dredmorbius]

Email round one: individually-watermarked copies are delivered to individually-addressed individuals. More elaborate systems might watermark such emails en-route, though that's ... less likely. A and B each wind up with individually-identifying copies of the email.

Email round two: A REPLIES ALL to their individually-watermarked copy of the email, delivering it to ALL employees (or some nontrivially large sample), by which B AND EVERY OTHER RECIPIENT now contains A's watermarked copy.

Email round three: B OR ANY OTHER RECIPIENT OF A's REPLY ALL can now leak A's watermarked copy of the email. Watermarking NO LONGER identifies the leaker.

[Shish2k]

> But how did recipient A get a personalised copy if the mail with that copy was sent to a list of recipients?

Watermarking added by the mailing-list manager software?