Postfix 架设的MAIL服务器，发hotmail的邮件发不出去，为什么？

大家好，我采用了有个提问的人说带个小附件发出去，搜狐，新浪都行，但hotmail的邮件发不出去，报错如下：This is the mail system at host mail.test.com.cn.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to <postmaster>

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

                   The mail system

<cindy_che950@hotmail.com>: host mx2.hotmail.com[65.54.244.168] said: 550
    DY-001 Mail rejected by Windows Live Hotmail for policy reasons. We
    generally do not accept email from dynamic IP's as they are not typically
    used to deliver unauthenticated SMTP e-mail to an Internet mail server.
    http://www.spamhaus.org maintains lists of dynamic and residential IP
    addresses. If you are not an email/network admin please contact your
    E-mail/Internet Service Provider for help. Email/network admins, please
    visit http://postmaster.live.com for email delivery information and support
    (in reply to MAIL FROM command)
这是为什么？还有就是为什么带附件发搜狐，新浪就行，不带搜狐，新浪的邮箱就收不到，这是为什么？非常感谢！