Cannot authenticate/activate iMessage account

Hi,

I have been trying to find a solution to the below error message for some weeks now. I have re-installed my mac, updated to the latest software, deleted pilst files that people have suggested and I have come to end of my tether with it! The account is still active and works, the details are correct but I am struggling to get the messages to come through. They are working on my iPhone but my iPad and Mac are refusing to sign in. Does anyone have any ideas as to what might cure this?

Thanks,

Luca