You can—and should—discard messages you do not want to keep. You can delete messages as you are reading them, or straight from a mail folder.

First select the messages you want to delete in the folder by holding down the Ctrl key and clicking to select the messages.

Then use the Delete Message command from the Edit menu or use the Delete button on the tool bar. You can also use the Del key on the keyboard.

Messages you delete are transferred into Mail and Newsgroups' Trash folder. You can get a message back from the Trash anytime until it is emptied, just click on the Trash icon.
 
 
 
 
 
 
 
 
 
 
 

You empty the Trash using the Empty Trash command from the File menu. Mail and Newsgroups never empties the Trash itself, so you should make a point of emptying the Trash regularly to free up disk space.