How to file a complaint

I have sent a complaint and it keeps coming back in my e mail unanswered no matter how many times I resent it,I need a working e mail to send my complaint asap,this is ridiculous that I can’t find a way to make a serious complaint,this is no different than the months I’ve spent calling Xfinity just to be called a liar from a manager,and now I can’t find anyone way to complain because I can’t get through to file a complaint.so please send me a real e mail to send.