E-mail Problem: ResponseCode 421, 4.7.1 Intrusion prevention active for

Dear all,

My partner is receiving a separate e-mail from Zoho each time he sends an e-mail. The e-mail is as follows:

From : mailer-daemon@mail.zoho.com<mailer-daemon@mail.zoho.com>
To :
Date :
Subject : Mail Delivery Status Notification (Delay)
============ Forwarded message ============

This message was created automatically by mail delivery system.

THIS IS A WARNING MESSAGE ONLY.

YOU DO NOT NEED TO RESEND YOUR MESSAGE.

The original message was received at Thu, 19 Oct 2017 03:07:55 -0700
from xxx

----- The following addresses had fatal errors -----
[Status: Error, Address: < xxx>, ResponseCode 421, 4.7.1 Intrusion prevention active for [xxx][D]]

Message will be retried for 4 more day(s)

----- Transcript of session follows -----

Received:from mail.zoho.com by mx.zohomail.com
    with SMTP id 1508404964118877.4212590600275; Thu, 19 Oct 2017 02:22:44 -0700 (PDT)
Message-ID:<xxxx>
Date:Thu, 19 Oct 2017 12:22:44 +0300
From:xxx>
User-Agent:Zoho Mail
To:xxx
Subject:xxx
Content-Type:xxx

______________

Any ideas how to fix this?

Thanks!