I guess it’s a variation of the reply-to-all-problem on mailing lists: when people receive an email from sender x, they assume that if they reply to this email, the reply will go (only) to sender x. With watching_first_post, discourse already has a great way of mitigating that behaviour (at least those replies will not be emailed to all), but I don’t see any additional steps I could take, apart from clarifying the email footer (which I have already done).
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