Mails are not getting delivered to Hotmail accounts due to the following error:
Message could not be delivered to some recipients. The following recipient(s) could not be reached: Recipient: [SMTP:firstname.lastname@example.org] Reason: 550 SC-001 (BAY004-MC1F30) Unfortunately, messages from 220.127.116.11 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.
The mail server's IP is blocked by Hotmail.
Hotmail is blocking the mails from the server 18.104.22.168 according to their spam policy. This can be caused as they found high volume of emails from your server.
Refer the following article to get more information regarding this error in
SMTP Error Codes section:
Mail rejected by Outlook.com for policy reasons. Reasons for rejection may be related to content with spam-like characteristics or IP/domain reputation. If you are not an email/network admin please contact your Email/Internet Service Provider for help.
To resolve this contact Hotmail support.