Mail delivery failed: returning message to sender

Hello,

I keep getting various email accounts sent back to me stating that delivery failed.

SMTP error from remote mail server after pipelined sending data block:
    550 5.7.1 Unfortunately, messages from [66.96.189.7] 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. [DB5EUR02FT031.eop-EUR02.prod.protection.outlook.com 2023-05-31T21:41:21.904Z 08DB61C0402EE890]