Microsoft randomly blacklists IP addresses for no reason whatsoever, making it
impossible to send email to hotmail.com and outlook.com from those IP
addresses. Each time this happens one has to contact Microsoft in order to get
de-listed.
I got so fed up with the whole thing that I blacklisted Hotmail and Outlook;
If I can't send email to Hotmail or Outlook, Hotmail and Outlook can't send
mail to me.