Backscatter StrangerNotAllowed spam mail generated from incoming category addresses

We’re using incoming category addresses on two of our categories, to allow users to start new threads by email. We handle incoming mail using the mail-receiver container. At the moment, we’re seeing a high level of spam from unregistered accounts to the incoming addresses - each of these generates a StrangerNotAllowed email reply to the spammer.

I note that in Reducing backscatter in email interface? the container was updated to do SMTP-time checks to Discourse to see if a post should be permitted - I’ve updated the mail-receiver container to 1.1.2 to enable this, and it has correctly started dropping backscatter generated by mail to invalid incoming email addresses.

However we’re still getting the StrangerNotAllowed backscatter from the valid incoming addresses, so I’m wondering if there’s options/hacks that could be done to make a similar SMTP-time check on this type of rejection, and not send any customised warning to the user. I appreciate this rejection mail has a function, so I guess it should be a setting? Perhaps a domain-list that should be dropped in this way?

Obviously if I’ve missed an existing way to handle this, then apologies and please enlighten me :smiley: