@zogstrip would you know what might be going on?
It seems like there used to be a check in the envelope in this commit: add support for incoming emails in CC/BCC fields · discourse/discourse@93d1cc6 · GitHub (specifically discourse/lib/email/receiver.rb at 93d1cc6294085008e025bb3032b231a6a81c6480 · discourse/discourse · GitHub) but then it seems to have been refactored in more recent version of receiver.rb.