Junk Mail is turned on by default when you start up Outlook. The default set to low, which catches the most obvious junk. If you leave it on, make sure you check the junk email folder to see what it is catching. The folder is located in the Navigation Pane under your cabinet folders. If you find it is collecting valid email, then you can turn it off, but keep in mind you will then receive all the junk mail. There are a few ways to combat this problem.
Note: Adding important addresses to your address book (Contacts) will prevent them from being sent to the Junk Mail folder.
Under the Level tab, you can set the level of sensitivity of the email filter. You can also set the number of days before the Junk Email folder will delete items.
Under the Blocked Senders tab, you will see emails that you blocked using the Block Sender in the drop down menu of the Junk Email button. You can also add any senders here that you would like blocked.