Have been trying to email several of our clients and many of our emails keep getting bounced back with an error that states:

ERROR CODE :550 - "JunkMail rejected - sender4-op-o16.zoho.com [136.143.188.16]:17694 is in
an RBL: Blocked - see spamcop.net/bl.shtml?136.143.188.16"

I searched this forum and found several posts about the same issue, the same IP address. It was marked as Solved, but that does not seem to be the case. Any assistance or guidance would be very helpful. 

Thanks,