Hi @Mascha Westen-Reinders Folmer ,
Glad to hear this issue has been resolved!
Thanks for the sharing.
However, due to a recent update in forum policy, the question author now is not able to accept their own answers.
So I have written a brief summary of the solution this issue. Please feel free to accept it as the answer, which would benefit others who also has similar issues in forum.
Issue:
cpanelemaildiscovery.cpanel.net what is this pop up?
Solution:
Problem seems to be solved by removing the email address in question and re-adding it to Outlook.