Auto Complete Help

Hi,

Every few months it seems my auto compete address list comes up blank. To
the best of my knowledge I am not purging the file contents or deleting the
nk2 file. Does any know;

Of a setting that would purge or delete this file automatically?
If a backup of the file is maintained?
A way to rebuild the file, short of sending everyone a email?
Could it have something to do with the Exchange server when picking up mail?

Thanks