Mails to certain emails blocked due to spamcop

I am having the following issues with at least 2 email addresses, both seem to be from mweb.co.za emails and from the IP address, 136.143.188.12

Error message:

This message was created automatically by mail delivery software.
A message that you sent could not be delivered to one or more of its recipients. This is a permanent error.
(email redacted)mweb.co.za, ERROR CODE :550 - rejected because 136.143.188.12 is in a black list at bl.spamcop.net

Blocked - see https://www.spamcop.net/bl.shtml?136.143.188.12

Please revise on how to fix this issue asap as we have to stay in contact with these clients.

Thank you!