> Whenever I enter a character in the TO: line on Outlook, a dropdown
> appears where I can just click on existing email addresses that are
> kept somewhere. Probably my address book.
Outlook doesn't have an address book. For an explanation of the
autocompletion feature, see this:
http://www.ingressor.com/autocompletetips.htm
> But, all of a sudden, it
> doesn't work anymore and I am not aware of any actions I took that
> caused this to stop happening. Can you help me?
There are several things that might have happened. Some will allow you to
recover the autocompletion cache and some will not. Post a complete
description of the symptoms.

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Brian Tillman [MVP-Outlook]