One of client's domain is blacklisted by windows live hotmail. the domain is
198.61.198.15
See below:
host mx1.hotmail.com[65.54.188.72] said: 550 SC-001
(BAY0-MC1-F1) Unfortunately, messages from 198.61.198.15 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. (in reply to MAIL
FROM command)
The domain is not blacklisted on any other blacklist. Please delist us as soon as possible.