I have strong opinions about people and their policies. I usually just end up saying them publicly, but if it's an ally I would say them privately to the person first, then disagree with them publicly.

Weirdly, the main complaint (that I wasn't supposed to see) was they felt I'd laid out my position too pedantically (apparently because I use footnotes in email? Which I've done for like 30 years, so maybe everyone already hates me for it and no one told me?)

I mean, I can be unreasonable, but don't mind folks discussing it frankly with me. But don't gossip!

Which is why I think @jwildeboer response will still work. It points out you've seen it without provoking any further. If it was intentionally public, you've just given the sender a huge shrug while moving on.

the thing to keep in mind is that a certain sort of ass does this kind of "mistake" deliberately, as a boundary-erosion tactic. so it's important to set the boundary that no, you will not be drawn into this fight.

Rather than argue with them I'd be tempted to turn this into a teaching moment, pointing out that when they talk about someone who's absent, they might consider doing so in the same way they would if that person were present. Because there's never any guarantee that what you write won't get back to them.

What a awful and common condition. Right down to the mix of bullshit and unpleasant maybe truth!

@bkuhn

In situations where someone behaves as if they are in private but they're not, I think it's polite to let them know. So if someone walks into a room you're in and starts changing their clothes, you might say, "I'm here," or "Do you want this room to yourself?"