How to Remove Your IP Address from the Hotmail/Outlook Blacklist
Your sender reputation and email performance are directly dependent on your sending workflow. If you have bad sending practices, it can lead to serious deliverability problems and ultimately land you in the spam folder. Likewise, safe senders with proper email authentication, positive reputation, and great list management, should have better deliverability and inbox placement.
In order to protect their users from unsolicited emails, mailbox providers developed blacklists to flag bad senders, somewhat like a blocked senders list.