I’ve been after this same functionality – mainly to send an autoresponse to people who might user our old mailman admin email addresses like list-unsubscribe, etc, to point them at the shiny new discourse that replaces the lists.
Not having found anything plug-and-play, I’ve got a usable solution that adapts the postfix in the mail-receiver container to automatically send a (rate-limited) response to anyone who emails a particular email address.
Because I’m not a postfix config guru I’ve only managed to get it working on a different subdomain than the discourse processed emails, which in our case is okay because all emails are coming through an institutional SMTP relay first, and that is separating out the discourse emails and forwarding them on to mail-receiver with the domain part of the email address changed (and since Discourse seems to pick up the email addres in the original To: field anyway this doesn’t matter to discourse). If an actual postfix guru can get this working just for a specific email address on the same domain that would be great!
It’s not written as a plugin and I might get round to doing that some time, but it’s not too difficult to install as there’s a setup.sh script that does (nearly) everything needed. It also does not actually use discourse, just the postfix in mail-receiver, so config within discourse is not yet possible.