Board Member E-mail Is Being Rejected

My Board President e-mail is being rejected. This is the first time this has happen. We need to communicate with these individuals. This is the message that I am receivig:

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. The following address(es) failed:

 

  rathead@live.com

    host live-com.olc.protection.outlook.com [104.47.74.33]

    SMTP error from remote mail server after pipelined end of data:

    550 5.7.1 Unfortunately, messages from [66.96.186.5] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3140). You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors. [BN8NAM04FT029.eop-NAM04.prod.protection.outlook.com 2023-03-02T14:05:00.336Z 08DB1AED67E6D63A]

Please help me resolve this issue. If you can't help me then at least give me some insight as to who can.

Thank you,