Outlook.com/hotmail.com/twitter.com mails are bounced with status 5.5.0, 553

Outlook.com/hotmail.com/twitter.com mails are bounced with status 5.5.0, 553

Hi,

Since about 2 days, mails originating from outlook.com/hotmail.com/twitter.com are blocked. People receive a Delivery Status Notification. Can you please look into this?

This is the response (I obfuscated the e-mail address):
  1. Final-Recipient: rfc822;f***@vee***.nl
  2. Action: failed
  3. Status: 5.5.0
  4. Diagnostic-Code: smtp;553 sorry, that address is not in my list of allowed recipients; no valid cert for gatewaying (#5.7.1)

Kind regards,
Fokko