@zogstrip would you know what might be going on?
It seems like there used to be a check in the envelope in this commit: https://github.com/discourse/discourse/commit/93d1cc6294085008e025bb3032b231a6a81c6480 (specifically https://github.com/discourse/discourse/blob/93d1cc6294085008e025bb3032b231a6a81c6480/lib/email/receiver.rb#L37-L40) but then it seems to have been refactored in more recent version of receiver.rb.