people at my org were gleeful when they learned they could hook LLMs into Slack. Even if we had some reliable, well-used signature system, I think people would just let AI use it to send emails on their behalf.

If the AI age has taught me anything, it's that most people do not care what their output is. They'll put their name on anything, taste or quality does not matter in the least. It's incredibly depressing.

That's a different problem though. It's doing it on their behalf, not on behalf of a scammer who's impersonating them.

Well we should treat that as their own output. If it's crap, treat it the same way you would if they produced the crap themselves.