Someone has informed me that they are unable to send e-mails to my hotmail account. Their address is not on my blocked list.

They receive the following message:

5.1.0 - Unknown address error 550-"SC-001 (COL0-MC1-F41) Unfortunately, messages from 202.180.66.194 weren't sent. Please contact your Internet service provider since part of their network is on our block list.