Fill out and submit this form: https://support.microsoft.com/en-us/getsupport?oaspworkflow=...

First I got this strange and confusing response:

    Dear x
    Please note that your ticket number is in the subject line of this mail.
    x.x.x.x
    Note: Errors are unlikely, however, if an error is indicated, please resubmit the specific IP or IP range.
    Thank you,
    Outlook.com Deliverability Support
    Please do not reply to this message as it is from an unattended mailbox.
    Any replies to this email will not be responded to or forwarded.
    This service is used for outgoing emails only and cannot respond to inquiries.

About an hour later I got another e-mail from them informing me that my IP address had been "conditionally mitigated", and I was then able to send e-mails to Hotmail.