Riviera Play Casino - Withdrawal not processed £1000

I requested a withdrawal for £1000 on Aug 14th. It has been pending since. I have tried to contact the casino by support but every time I receive this message

Your message wasn’t delivered. Despite repeated attempts to deliver your message, the recipients email system refuse to accept a connection from your email system.

I have tried live chat but there is never anybody available to talk and I am told to leave an email and someone would get back to me. I have done this 3 times and nobody has ever got back to me.

The account manager Mat keeps sending me promotional emails. I have contacted him to chase my withdrawal but no response.

There is no phone number to contact the site.

I am getting really frustrated now as I gambled and won this fairly with no sign of withdrawal and nobody I can contact to help. Could you please look into this for me