"Diagnostic-Code: 5.2.1 Too many failures for the recipient"

I tried to send an email to a regular client today with a standard .doc attachment that their email client rejected with this message:

ERROR CODE :552 - 5.7.0 This message was blocked because its content presents a potential
            5.7.0 security issue. Please visit
            5.7.0 https://support.google.com/mail/?p=BlockedMessage to review our
            5.7.0 message content and attachment content guidelines. ga18-20020a17090b039200b001d988ce211asi3170091pjb.51 - gsmtp

After trying to resend, I am now getting a permanent error when I try to send anything to their address (with or without an attachment) as follows:

Status: 554
Action: failed
Last-Attempt-Date: 4 May 2022 00:38:57 GMT

Diagnostic-Code: 5.2.1 Too many failures for the recipient

I am able to receive messages from them and send and receive from all other email accounts. Can someone please assist urgently?