The MAIL server set up by Postfix cannot send hotmail emails. Why?

The MAIL server set up by Postfix cannot send hotmail emails. Why? -- Linux Enterprise Application-Linux server application information. For details, refer to the following section. Hello everyone, I used a person who asked a question to send it out with a small attachment. Both Sohu and Sina can do it, But hotmail cannot send it out. The following error is returned: this is the mail system at host mail.test.com.cn.

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

For further sort ance, please send mail

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

The mail system

: 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)
Why? There is also why Sina cannot receive the attachments sent to Sohu without Sohu. Why? Thank you very much!