Hello there, LoesEkk.
This is occuring because you likely have the Mail Preview pane open. Because it is "previewing" the email for you, it considers that as read. You can change that behavior by hiding the preview pane. Follow the steps outlined in the following Knowledge Base Article:
The preview pane, the lower portion of the Mail viewer window, allows you to view a selected email without opening it in a separate window.
The preview pane is visible by default, but you may hide it by double-clicking the separator. You can adjust the size of the preview pane by dragging the separator up or down.
If the preview pane is hidden, or if its size is reduced, it may not be visible.
When shown, the mailboxes drawer appears on the left or right side of the Mail viewer window. You can show or hide it by clicking the Mailboxes button in the Mail viewer window. You can also hide the mailboxes drawer by dragging its outer border toward the Mail viewer window.
Thanks for reaching out to Apple Support Communities.