Re-posted from Microsoft http://office.microsoft.com/en-us/outlook-help/about-the-junk-e-mail-filter-HP005242967.aspx

About the Junk E-mail Filter

The Junk E-mail Filter in Outlook is turned on by default, and the protection level is set to Low. This level is designed to catch only the most obvious junk e-mail messages. You can make the filter more aggressive, but if you do it may catch legitimate messages sometimes. Any message that is caught by the Junk E-mail Filter is moved to a special Junk E-mail folder. You should review messages in the Junk E-mail folder from time to time to make sure that they are not legitimate messages that you want to see.

 Note   Text in this topic that is marked with a single asterisk ( ) indicates a feature that is introduced with Microsoft Office 2003 Service Pack 1. Text that is marked with two asterisks ( ) indicates a feature that is introduced with Microsoft Office 2003 Service Pack 2. Starting with Microsoft Office 2003 Service Pack 2, messages that are moved to the Junk E-mail folder are automatically converted to plain text format, and any links that they contain are disabled. Any message that you move out of the Junk E-mail folder has its links enabled and the original message format restored, unless the message is considered by the Junk E-mail Filter to contain suspicious links. In that case, even if you move it out of the Junk E-mail folder, the links in the message remain disabled by default. For details, see Block suspicious messages and phishing schemes.

Choose any of the topics below to learn more about how to use the various functions of the Microsoft Outlook Junk Mail Filter:

• Add a name to the Blocked Senders List
• Add a name to your Safe Senders or Safe Recipients List
• Block or unblock links in suspicious phishing messages
• Change the Junk E-mail Filter options
• Edit the Junk E-mail Filter Lists

For more information on using the Microsoft Office Junk E-Mail Filters, go to http://office.microsoft.com/en-us/outlook-help/about-the-junk-e-mail-filter-HP005242967.aspx