If you ever encounter the following message it will be safe to say your servers ip has been blacklisted

550 SC-004 Mail rejected by Outlook.com for policy reasons. A block has been placed against your IP address because we have received complaints concerning mail coming from that IP address. We recommend enrolling in our Junk Email Reporting Program (JMRP), a free program intended to help senders remove unwanted recipients from their email list. If you are not an email/network admin please contact your Email/Internet Service Provider for help.

1.) Review Server Logs and or Scripts to identify any possible malicious content such as spam being sent out via a compromised script or a compromised account. If the reason for the spam is not removed before submitting a blacklist removal request, you will be blacklisted again and for a longer period.

2.) Submit a request to Hotmail by following this link https://support.microsoft.com/en-us/getsupport?oaspworkflow=start_1.0.0.0&wfname=capsub&productkey=edfsmsbl3 and

  provide your name, best contact email, what service you are sending to (live.com ; hotmail.com ; outlook.com or msn.com) the ip address of your server and the error message you are receiving. 
  

3.) Wait for a response from hotmail. Most responses take anywhere from 30mins to several hours. Once you receive a response hotmail will outline the action which has been taken and in most cases your blacklist will have been lifted. If your blacklist has not been removed you will need to resolve anything they have identified or reply back to them asking why it cannot be lifted and they will provide you with a reason.