Select the email to delete then right click -> delete or delete button in the bar at the top.
If you made a mistake, do not panic! You will find the email in the trash so it will be possible to recover it. Open your trash, right click on the mail and click on “move” select the folder you want it to go and click ok. If you want to permanently delete this email, you will have to delete it from the trash folder.
The trash
On the small arrow to the right of the trash click on “recover deleted items” or “empty trash”. Warning: empty trash erases ALL mails in the trash. To delate one by one, open the trash, select the mail you want and click on “delete”.