Emails send from a valid user are getting rejected with the following message


Hi. This is the qmail-send program at rediffmail.com.
I'm afraid I wasn't able to deliver your message to the following addresses.
This is a permanent error; I've given up. Sorry it didn't work out.

Connected to 65.54.188.110 but sender was rejected.
Remote host said: 550 SC-001 (BAY0-MC3-F10) Unfortunately, messages from 114.31.224.125 weren't sent. Please contact your Internet service provider since part of their network is on our block list. You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors.

I have checked the senders email is not present in block senders list. In-fact it is present in safe senders list.
Why the emails getting rejected?