Hi @CL ZRTI ,
Welcome to our forum.
I understand that you are working with Microsoft 365 account in Outlook 2016, is it right? If so, what's the version of your Outlook 2016(File>Office account>about outlook), please first ensure that you have updated to the latested version of Outlook.
As I know, we could usually search the message from a specific users via type From: "sender name" in search field, which has been declared in the official documentation: Search reference tables. Due to I haven't had a Dutch version of Outlook , I have just tested on my English client and found it could work normally there, it's suggested to kindly check if it could also work for you.
Also, for your requirement of searching the messages from a specific sender, as a easier way, we could also search the message via click the "From" button like below, which may help us search these messages more fast.
Hope that would be helpful to you.
If the response is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.