Received a message that Email::Receiver::TooShortPost but message is long

I’m setting up a new forum and trying to add a category with an ‘email-in’. In my testing I’m sending a rather length email but it is being rejected with the message ‘Email::Receiver::TooShortPost’ - looking for some pointers on how to see why it isn’t seeing the full email and getting a false positive on the error.