"When I delete messages, they don't go to the Trash
If your deleted messages aren't moved to the Trash mailbox, you haven't set that option in Mail Preferences. You can set deleted messages to move to the Trash mailbox, and you can specify how often messages in the Trash are permanently erased.
Open Mail > Preferences and click Accounts.
Select an account and click Mailbox Behaviors.
Click “Move deleted messages to the Trash mailbox.”
From the bottom pop-up menu, choose to permanently erase deleted messages when they are one day old, one week old, one month old, never, or when you quit Mail."
uit het Help menu van Mail