Microsoft Outlook uses Security Zones to provide control over how much
access incoming e-mail messages and Web pages have to your computer. These
security zones provide advanced protection against certain types of files
that may gain access to your computer and replace or damage files. You can
choose a security zone in which to place your incoming e-mail messages
based upon their source and you can select specific levels of security for
each zone.
On the Security tab, click Zone Settings, and then click OK.
Click to select one of the four zone icons, and using the slider
control under "Security level for this zone," select the desired level.
If you select Custom level, you must then click to select the desired
settings from the Security Settings dialog box.