Undelivered Mail Returned to Sender

Undelivered Mail Returned to Sender

Hi, I've received the following when sending mail to one of my regular recipients. 

ERROR CODE :550 - "JunkMail rejected - sender4-of-o51.zoho.com [136.143.188.51]:21128 is in an RBL on rbl.websitewelcome.com, see Blocked - see  http://www.spamcop.net/w3m?action=checkblock&ip=136.143.188.51"


See attached screenshot from the link above.