From the Computer Club's Help Center:
If you are using an email client (e.g., iPhone/iPad mail app, Microsoft Outlook, Thunderbird) and are unable to send/receive Outlook.com or Hotmail.com emails – surprise! – Microsoft has upgraded security for these email addresses and you must take the actions provided here:
If you are using eM Client, be sure to use the most recent version. You may have to delete and re-enter your Outlook/Hotmail account.
Make an appointment with the Help Center if you are unable to access your mail.