There are two ways to delete a message, one from the mailbox and one from the message window . Both are covered in this tutorial.
Any deletion is a two step process in Mulberry.
Open a mailbox, highlight the message with one mouse click as illustrated above Click on the Delete icon in the icon bar. Message will appear as illustrated above with red line and red X through the message You can delete a message from inside the message box. Click on the Delete Message and Read Next Message icon.