Disable Outlook's address-autocomplete feature
I didn't realize how much I had come to rely on Microsoft Outlook's ability to automatically complete the e-mail addresses I entered in the To:, Cc:, and Bcc: fields until a recent Microsoft Exchange server update at my office wiped out the entries. Of course, one person's convenience is another person's security risk.
Eli Lilly and Co. found this out the hard way last year after a lawyer in the company's employ sent a confidential memo intended for a colleague to a report for the New York Times whose name was similar to the coworker's.
To disable Outlook's address-autocomplete feature, click Tools > Options > E-mail Options (under the Preferences tab) > Advanced E-mail Options. Uncheck "Suggest names while completing To, Cc, and Bcc fields" and click OK three times.
Block Outlook from autocompleting addresses by unchecking this setting in Advanced E-mail Options.
(Credit: Microsoft)If you're a fan of Outlook's autocomplete feature, you may want to export your autocomplete entries to another PC. Microsoft provides instructions for doing so, though Vista users will need to refer to one of the article's comments to find the location of the .nk2 file they need to export.
But there's a better way: Nirsoft's free NK2View lets you view the entries in this file and export them as a text file, HTML, or XML. You'll find more information about the utility on the NirSoft site.
Dennis O'Reilly has covered PCs and other technologies in print and online since 1985. Along with more than a decade as editor for Ziff-Davis's Computer Select, Dennis edited PC World's award-winning Here's How section for more than seven years. He is a member of the CNET blog Network, and is not an employee of CNET. 
Turning off autocomplete for mail (for me) seems as limiting as turning off scripting for web browsing. Sure, it's safer, but what about how it limiting it is to your every day functionality.