Error message in his iCloud Email box - unable to send.

My father keeps receiving an error message in his iCloud Mailbox along the lines of the following.

Begin forwarded message:

From: Mail Delivery Subsystem <mailer-daemon@googlemail.com>

Subject: Delivery Status Notification (Failure)

Date: November 2, 2020 at 5:31:50 AM EST

To: redacted+caf_=redacted=icloud.com@gmail.com

Message blocked

Your message to ***@icloud.com has been blocked. See technical details below for more information.LEARN MORE

This link will take you to a third-party siteThe response from the remote server was:

554 5.7.1 [CS01] Message rejected due to local policy. Please visit https://support.apple.com/en-us/HT204137

Reporting-MTA: dns; googlemail.com

Received-From-MTA: dns; SRS0=/qzm=EI=mx.sailthru.com=delivery_20201102053134.21968270.9527@redacted.com

Arrival-Date: Mon, 02 Nov 2020 02:31:39 -0800 (PST)

X-Original-Message-ID: <20201102053134.21968270.9527@sailthru.com>

Final-Recipient: rfc822; redacted@icloud.com

Action: failed

Status: 5.7.1

Remote-MTA: dns; mx02.mail.icloud.com. (17.42.251.12, the server for the

domain icloud.com.)

Diagnostic-Code: smtp; 554 5.7.1 [CS01] Message rejected due to local policy. Please visit https://support.apple.com/en-us/HT204137

Last-Attempt-Date: Mon, 02 Nov 2020 02:31:50 -0800 (PST)

What does this mean?