Unexplained Email::Receiver::InvalidPost errors

Oh that’s interesting, and seems to be contrary to the guidance posted here to setup a mailing list mirror:

Subscribe to the mailing list using your Discourse email address (e.g., discourse@example.com).

I checked the email address subscribed to our mailing lists on behalf of Discourse and it’s indeed discourse@forum.torproject.org.

Should I be using a different email address here? If so, which one?