Zoho mail server (IP) treated as spam, the email cannot be delivered to the recipient

Zoho mail server (IP) treated as spam, the email cannot be delivered to the recipient

When we try to send out to a customer, it return:
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.

xxx@customer_email.com INVALID_ADDRESS



Reporting-MTA: dns; mx.zohomail.com
Arrival-Date: Thu, 22 Apr 2021 14:58:55 +0200

Original-Recipient: rfc822; xxx@customer_email.com
Final-Recipient: rfc822; xxx@customer_email.com
Status: 550
Action: failed
Last-Attempt-Date: 22 Apr 2021 12:59:06 GMT
Diagnostic-Code: spamcop.mimecast.org Blocked - see https://www.spamcop.net/bl.shtml?136.143.188.16. - https://community.mimecast.com/docs/DOC-1369#550 [rI3TmyzeMzympXglxVKHSg.za137]

What can we do?