To change the junk mail filter:

Choose Mail > Preferences, and then click Junk Mail.

Specify which actions Mail should perform when junk mail arrives:
When you want to verify what the Mail is identifying as junk mail, select the “Mark as junk mail, but leave it in my Inbox” checkbox.
When you are sure the filter accurately identifies junk mail, select the “Move it to the Junk mailbox” checkbox.
To set up other actions, select the “Perform custom actions” checkbox and then click the Advanced button. In the dialog that appears, click Add (+) buttons to add conditions and actions. To remove a condition or action, click its Remove (-) button.
Note: To use the junk mail database when identifying junk mail, make sure “Message is Junk Mail” is one of the conditions (it’s specified by default). Otherwise, Mail uses only the conditions listed to identify junk mail.

Indicate the types of messages that don’t need to be evaluated by the junk mail filter. Typically, these are messages from senders you know.

To include your ISP’s junk mail detection in the filter, select “Trust junk mail headers set by my Internet Service Provider.”