Users who are using the Zimbra Connector for Outlook (ZCO) may, at times, receive a GAL Reset recommended pop-up alert that states "It is recommended that you reset your Global Address List. Do you wish to proceed now?"
During the startup of Outlook, the connector will validate the difference between the Global Address List on the server and locally within OL. If there are significant differences, the server will send a fullSyncRecommended request to the ZCO that will trigger the recommendation to appear.
Resetting the GAL will not replace the existing Global Address List until the sync has fully completed. If the sync does not completed before the client is taken offline, the user will receive the recommendation again. The sync time will vary based on the size of the GAL and network speed.
To resolve the issue, simply select Yes within the pop-up or you can take the following steps within Outlook:
Select the Zimbra Tab to display the Zimbra ribbon
Next select Sync Global Address List drop down menu and choose the Reset Global Address List option.