First you will need to fill the auto-complete cache
You can directly fill the list again by creating a new message addressed to all your contacts at once. There is no need to actually send this message.
Create a new message.
Press the To… button.
Select the contact list in the right top corner from which you would like to add the contacts from.
Examples; Contacts folder, Global Address List (GAL) or a contacts folder in a Public folder.
Select the first contact.
Scroll all the way down to your last contact.
Hold the SHIFT button on your keyboard while clicking on the last contact to select them all.
Press the To –> button to add them all to the To… field.
Repeat step 3 to 7 if you have contacts in additional lists that you would like to add.
Press OK to close the dialog and to return to the newly created message.
If you are using Outlook 2007 or previous, close the message and press No when prompted to save changes.
For Outlook 2010 and Outlook 2013, you must send the message to actually store these addresses in the AutoSuggest cache. However, by
placing Outlook in Offline Mode first via the Send/Receive tab, you can easily prevent it from actually being sent by removing it from the Outbox and then place
Outlook back online mode.
On the File menu, click Work Offline.
To set up an Offline Folder file (.ost), click OK.
By default, the Prompt me at startup so I may choose to work offline or online check box is selected. If you want Outlook to always work online when a connection is available, clear this check box.
After the .ost file is created, when you exit and restart Outlook, you must synchronize your Exchange mailbox with the .ost file. The fastest way to do this is as follows: On the
Tools menu, point to Send/Receive, and then click Send/Receive All.
Thanks to http://www.msoutlook.info/question/216
Technical Support Engineer
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