[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.