Email to icloud.com email address rejected (The following organization rejected your message: mx1.mail.icloud.com)

Hi,

I can email my own email address from work and it is received with no problems.

However when I email my wife's icloud address it is rejected. The failure message is pasted below, and I would appreciate any suggestions. Many thanks. (NB I did initially report to my organisation's IT dept who weren't interested).

Delivery has failed to these recipients or distribution lists:

(emailredacted)@icloud.com

Your message wasn't delivered because of security policies. Microsoft Exchange
will not try to redeliver this message for you. Please provide the following
diagnostic text to your system administrator.

The following organization rejected your message: mx1.mail.icloud.com.

Diagnostic information for administrators:

Generating server: smtp1.e.amses.net

(emailredacted)@icloud.com

mx1.mail.icloud.com #<mx1.mail.icloud.com #5.7.0 smtp; 550 5.7.0 Blocked -
see https://support.proofpoint.com/dnsbl-lookup.cgi?ip=213.161.89.71>
#SMTP#