I been getting emails for years from the same sender.  Today I received an email from them that they were getting the email back with this error:

550 SC-001 (SNT0-MC1-F28) Unfortunately, messages from 74.121.38.132
weren't sent. Please contact your Internet service provider since part of
their network is on our block list.

Please do not tell me to contact the domain administrator.  There is no one to contact! I have looked at many of the discussions and none of them make any sense and do not tell you how to fix the problem.  How do I fix it so they can email me.? These are very important emails that I can't not get!