INVALID_ADDRESS, ERROR_CODE :550, ERROR_CODE :"JunkMail rejected - sender4-of-o51.zoho.com [136.143.188.51]:21108 is in an RBL: Blocked

INVALID_ADDRESS, ERROR_CODE :550, ERROR_CODE :"JunkMail rejected - sender4-of-o51.zoho.com [136.143.188.51]:21108 is in an RBL: Blocked

Can't send mail to one of my clients,

​

INVALID_ADDRESS, ERROR_CODE :550, ERROR_CODE :"JunkMail rejected - sender4-of-o51.zoho.com [136.143.188.51]:21108 is in

            an RBL: Blocked - see https://www.spamcop.net/bl.shtml?136.143.188.51"