Relating to this topic, it seems to me my problem would mostly go away if discourse simply ignored emails from blocked users or blacklisted domains. Currently it responds with an email notification, generating tens of thousands of needless emails and triggering mailgun’s warnings. (I have no evidence to prove this, but I suspect these responses also encourage the spammers by showing a sign of life)
This is a bit draconian and only matters for sites where topic creation is allowed through email, so it could be an admin setting.