Trying to log a complaint with Discovery is practically impossible

Despite my forwarding claims they are not been acknowledge. I contacted Discovery and asked for the email for their Complaints Department and was told its ***. I sent an email to this address and received the following.... our message to *** couldn't be delivered. The group complaints only accepts messages from people in its organization or on its allowed senders list, and your email address isn't on the list.

This is the first of two complaints the second one being more serious but I have no idea who I should send it to as Discovery's Complaints Department email seems to be their best kept secret. I have spent 30 mins looking for a complaints email on their website but without success.