Mail opens new window

Hello.

I recently bought a MacBook Pro 2020 in exchange of my 2015 Pro.

Before in the 2015, when i opened an email it would open the email within the same program/page. Allowing easy access to different mails, clicking between them. It would simply open it within the mail program and have the email on the right side.

Now on my new 2020, I have to double click and it opens the mail in a separate window. I cannot find the setting to turnt his off - is it not possible, to have the email open within the program and not in a separate window?