When you receive an outlook invitation you might receive the following message:

  “A custom mail flow rule created by an admin at wittenborg.eu has blocked your message.”

Any time you receive an invitation in Microsoft Outlook or 365 and accept it, the invitation will be visible from then on in your calendar. Therefore, even if you close the window that pops up while you have accepted the invitation, you can always go to your Outlook calendar and join the communication channel again.

Note:

If the problem persists you may contact Helpdesk for further assistance.